Here is a topic: http://stackoverflow.com/questions/2507734/cant-get-up-with-ria-demo where I've described problem with getting access to DomainContext object on the client. That time problem was resolved himself (without any actions from my side, with Visual Studio restart).

At the moment, I had the same problem on another PC (Vista x64, VS2008, Silverlight 3.0, RIA Services Toolkit): ThomasDomainContext class that inherits DomainContext was not visible from the page's codebehind file.

Again, it was resolved after VS restart and after that I got another problem:

Error   1   'VfmElita.Web.ClientBin.ThomasDomainContext' does not contain 
a definition for 'employee' and no extension method 'employee' accepting a first 
argument of type 'VfmElita.Web.ClientBin.ThomasDomainContext' could be found 
(are you missing a using directive or an assembly reference?)
D:\Project\Budda\VFMElita\VfmElitaSilverlightClient\MainPage.xaml.cs

for the following lines of code (error is noticed for the 31st line):

    30:    var context = new ThomasDomainContext();
    31:    grid1.ItemsSource = context.employee;
    32:    context.Load(context.GetEmployeeQuery());

Any help is welcome.

P.S. Please note, that source code create on the 1st machine is successfully compilable and launchable