Two machines. Both with .NET 3.5 and the VS 2008 VC++ SP1 redistributables
A single exe which uses two signed DLLs, one in C++/CLI and one in C#
The exe load
You might be able to bypass this on development by going into the project settings -> Signing -> and unchecking "Sign the assembly".
I ran into this today while debugging against a source code copy of the Entity Framework.