As I was building some new customer FIM 2010 workflows for a customer, I ran into that little annoying feature called Blocked File Protection Control.
I had created my initial Workflow Activity Library for FIM and references the FIM binaries to get access to the FIM activities. As I build my project I received a compiler error saying "Compilation failed. Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information."; this had me for several hours.
It turned out that since I had copied the FIM binaries from the FIM server to the Windows 7 developer computer, the OS had detected these files as potential "harmful" (FIM 2010 harmful :-) ?) and blocked these files. The error message that I got from the compiler was very helpful, so it actually took me a few hours to realize what was going on. After unblocking the DLL's I was able to compile.
If you run into this problem, check if the DLL's are blocked; in Explorer, goto Properties on the file and check the general tab for the text "This file came from another computer and might be blocked to help protect this computer".