Last week I did some test with the new Silverlight 5 functionality. Afterwards I reverted the solution back to Silverlight 4. But from that moment on, compilation started to fail with the following error messages:
The strange thing was that all the necessary references were available. After trying a lot of things, I was able to solve the problem in the end by removing my userfile (*.suo) from the solution directory. Visual Studio recreated the file the next time I opened the solution and everything was working again.