Here is a quick update of what I've done so far:
1) I downloaded the ZIP source file (
http://hibernatingrhinos.com/downloads/rhino-mocks/latest)
2) Tried to build it but couldn't since all the AssemblyInfo.cs files were missing.
3) Created an empty Assemblyinfo.cs files for all projects. Removed the options to storngly sign the assemblies.
4) Had to comment or change 3 test cases (Rhino.Mocks.Tests project) and 1 test-related file (Rhino.Mocks.Tests.Model) as a side-effect of not strongly signing the assemblies and conversion to .NET 4.0.
In the end, it worked, and the error I was getting is now gone. Hurray!
I suggest that someone really involved in the project moves ahead and does a proper strongly-signed build of it, since it's apparently not that difficult. And would reduce his karma. :-)