If u are running VS2010 beta 2 and using Silverlight 4 then don't install the Release Candidate!!
http://blogs.msdn.com/wpfsldesigner/archive/2010/02/09/silverlight-support-for-visual-studio-2010-release-candidate.aspx
Quote from Karl Shifflett
Silverlight 3 projects are supported with the Visual Studio 2010 Release Candidate build – however Silverlight 4 projects are not yet supported.
We will be adding Visual Studio 2010 Release Candidate support for Silverlight 4 with the next public Silverlight 4 drop.
If you are doing active Silverlight 4 development today we recommend staying with the Visual Studio 2010 Beta 2 build for now.
Have a great day,
Karl Shifflett
Visual Studio Cider Team
Glad I saw your post. Thanks for the heads up!
Yes, sadly this means CSLA 4 work is stuck on beta 2 for the moment 
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