I have a ubr file setup to stress test an internal DotNetNuke site with WCAT:

transaction
{
    id = "Intranet Home Page";
    weight = 1000;

    cookies{clear = true;}
    sleep{delay = rand("1","500");}

    request
    {
        url         = "/";
        statuscode = 401;
    }

    request
    {
        url         = "/";
        authentication = ntlm;
        username = "mydomain\\accountname";
        password = "password";
        statuscode = 200;
    }

    close{ method = reset;}
}

When running this (wcat.wsf -run -clients localhost -s myserver -t test.ubr -f settings.ubr -x) I simply get lots of error 500s:

2010-03-08 10:29:31 192.168.11.239 GET / - 80 - 192.168.52.139 - 401 2 2148074254 2010-03-08 10:29:31 192.168.11.239 GET / - 80 - 192.168.52.139 - 401 1 0 2010-03-08 10:29:31 192.168.11.239 GET /Default.aspx - 80 mydomain\myaccount 192.168.52.139 - 500 0 0

DNN is reporting these errors as:

AssemblyVersion: 5.2.3
PortalID: 0
PortalName: My Company
UserID: -1
UserName:
ActiveTabID: 39
ActiveTabName: Home
RawURL: /Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider ExceptionGUID: 28d8821f-1ef2-41db-8a65-d33e97a69130
InnerException: *Unhandled Error:*
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule.OnAuthenticateRequest StackTrace:
Message: System.Exception: Unhandled Error:
---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule.OnAuthenticateRequest(Object s, EventArgs e) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) --- End of inner exception stack trace ---
Source:
Server Name: MYSERVER

It seems to be losing the username somehow.