When I had installed the SP 1 of Visual Studio 2010 the caching of my built libraries got screwed up.
I had to manually right click and choose rebuild on every single library instead of just using Ctrl+Shift+B. Really strange. Even after de-installing SP 1 the problem remained until I manually recreated the entire solution – problem solved.
Might be unrelated to the SP 1 install of course, just a bad timing but since SP 1 really didn’t speed things up I can’t see any reason to reinstall it.