Another thing to try is to make sure your Copy Local for all custom DLLs in your web project are set to True. I had them set to false because i wanted the project to reference the DLLs from a network location, and things worked fine for a while, but mysteriously, i got the dreaded "Could not load type...' error for all user controls and web pages. When i flipped them back to True, the project could then load the UI types. So obviously there's some DLL reference mapping issue (whether intended or not, i don't know) in VS. I use VS 2008 SP1 with .Net 3.5.