Migrate from Silverlight 2 to 3
Just a quick note on this as I still found people stuck with this error:
Error: Unhandled Error in Silverlight 2 Application
Message: Failed to load the application. It was built with an obsolete version of Silverlight
This might happen every time you download a sample which was build with Silverlight 2, and you open it in VS2008 updated to Silverlight 3.
The solution is very easy:
Delete the xap file in ClientBin folder and rebuild the Silverlight project.