I've reproduced this in a simple project - details below. It's a WCF service in ASP.NET compatibility mode. What I'm seeing is that when run on the dev machine (Win7), a HTTP session id is available inside the service operation (HttpContext.Current.Session is non-null). But when deployed to the server (Win2k8R2), I get "No session". On both machines the app is configured to use the classic app pool, and the app pools themselves are configured identically as far as I can tell.

The only differences I can discern between the two applications is that on the dev box, under "Handler Mappings", ISAPI-dll is disabled (not on the server), and on the server there's a spurious handler called "AboMapperCustom-7105160" (does not exist on the dev box).

What should I be looking at next? Am I missing something head-slappingly simple?

Service is this:

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class Service2 {

    [OperationContract]
    public string DoWork() {
        if (HttpContext.Current != null) {
            if (HttpContext.Current.Session != null) {
                return "SessionId: " + HttpContext.Current.Session.SessionID;
            }
            else {
                return "No Session";
            }
        }
        else {
            return "No Context";
        }
    }
}

Config is:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <configSections>
      <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net, Version=1.2.9.0, Culture=neutral, PublicKeyToken=b32731d11ce58905" />
      <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
        <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
          <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication" />
          <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
            <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="Everywhere" />
            <section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication" />
            <section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication" />
            <section name="roleService" type="System.Web.Configuration.ScriptingRoleServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication" />
          </sectionGroup>
        </sectionGroup>
      </sectionGroup>
    </configSections>

  <log4net>
    <appender name="LogFile" type="log4net.Appender.RollingFileAppender">
      <file value="C:\Temp\Test.log4net.log" />
      <rollingStyle value="Once" />
      <maxSizeRollBackups value="10" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%d{ISO8601} [%5t] %-5p %c{1} %m%n" />
      </layout>
    </appender>
    <root>
      <level value="DEBUG" />
      <appender-ref ref="LogFile" />
    </root>
  </log4net>
  <appSettings />
    <connectionStrings />
    <system.web>
      <compilation debug="true">

          <assemblies>
            <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
            <add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
            <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
            <add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
          </assemblies>

        </compilation>
        <!--
            The <authentication> section enables configuration 
            of the security authentication mode used by 
            ASP.NET to identify an incoming user. 
        -->
        <authentication mode="Windows" />
        <!--
            The <customErrors> section enables configuration 
            of what to do if/when an unhandled error occurs 
            during the execution of a request. Specifically, 
            it enables developers to configure html error pages 
            to be displayed in place of a error stack trace.
        -->

            <customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
            <error statusCode="403" redirect="NoAccess.htm" />
            <error statusCode="404" redirect="FileNotFound.htm" />
        </customErrors>

      <pages>
        <controls>
          <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
          <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        </controls>
      </pages>

      <httpHandlers>
        <remove verb="*" path="*.asmx" />
        <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" validate="false" />
      </httpHandlers>
      <httpModules>
        <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      </httpModules>

    </system.web>

    <system.codedom>
      <compilers>
        <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
          <providerOption name="CompilerVersion" value="v3.5" />
          <providerOption name="WarnAsError" value="false" />
        </compiler>
     </compilers>
    </system.codedom>

    <!-- 
        The system.webServer section is required for running ASP.NET AJAX under Internet
        Information Services 7.0.  It is not necessary for previous version of IIS.
    -->
    <system.webServer>
      <validation validateIntegratedModeConfiguration="false" />
      <modules>
        <remove name="ScriptModule" />
        <add name="ScriptModule" preCondition="managedHandler" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      </modules>
      <handlers>
        <remove name="WebServiceHandlerFactory-Integrated" />
        <remove name="ScriptHandlerFactory" />
        <remove name="ScriptHandlerFactoryAppServices" />
        <remove name="ScriptResource" />
        <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      </handlers>
    </system.webServer>
    <runtime>
      <assemblyBinding appliesTo="v2.0.50727" xmlns="urn:schemas-microsoft-com:asm.v1">
        <dependentAssembly>
          <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" />
          <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
        </dependentAssembly>
        <dependentAssembly>
          <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35" />
          <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
        </dependentAssembly>
      </assemblyBinding>
    </runtime>

  <system.serviceModel>
        <bindings>
          <basicHttpBinding>
            <binding name="BasicHttpBinding_Service2" maxBufferSize="2147483647" maxReceivedMessageSize="2147483647">
              <security mode="TransportCredentialOnly">
                <transport clientCredentialType="Windows" />
              </security>
            </binding>
          </basicHttpBinding>
        </bindings>
        <serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
        <behaviors>
            <serviceBehaviors>
                <behavior name="WebApplication3.Service2Behavior">
                    <serviceMetadata httpGetEnabled="true" />
                    <serviceDebug includeExceptionDetailInFaults="false" />
                </behavior>
            </serviceBehaviors>
        </behaviors>
        <services>
            <service behaviorConfiguration="WebApplication3.Service2Behavior" name="WebApplication3.Service2">
                <endpoint address="" binding="basicHttpBinding" bindingConfiguration="BasicHttpBinding_Service2" contract="WebApplication3.Service2" />
            </service>
        </services>
    </system.serviceModel>

  <system.diagnostics>
    <sources>
      <source name="System.ServiceModel"
              switchValue="Information, ActivityTracing"
              propagateActivity="true">
        <listeners>
          <add name="traceListener"
               type="System.Diagnostics.XmlWriterTraceListener"
               initializeData="c:\Temp\Test2.svclog" />
        </listeners>
      </source>
    </sources>
    <trace autoflush="true"
           indentsize="4">
      <listeners>
        <add name="traceListener2"
             type="System.Diagnostics.TextWriterTraceListener"
             initializeData="c:\Temp\Test.log"
             traceOutputOptions="DateTime" />
      </listeners>
    </trace>
  </system.diagnostics>
</configuration>

Testing with a simple console app:

class Program {
    static void Main(string[] args) {
        ServiceReference1.Service2Client client = new ServiceReference1.Service2Client();
        Console.WriteLine(client.DoWork());

        Console.ReadKey();
    }
}