I ran into this after having Visual Studio convert my Beta 1 project to Beta 2.
System.Security.VerificationException was unhandled by user code
Message="Operation could destabilize the runtime."
I created a new Beta 2 solution from scratch and noticed at least one difference, which turned out to be a fix in my case. The reference to mscorlib was pointing to "%\Program Files\Microsoft Silverlight\2.0.30226.2\mscorlib.dll" for my converted project, but was pointed to "%\Program Files\Microsoft SDKs\Silverlight\v2.0\Reference Assemblies\mscorlib.dll" in the new Beta 2 solution. I updated the reference to the new location and all was well except for a few Beta 2 breaking changes that were easy to fix.
Note: I had two references to mscorlib in the previous location, inside two projects in my solution. One was a Silverlight Library, and the second was a Silverlight App that referenced the library. The error stack had the error inside the referenced library.