Ugh! I was experiencing the exact same symptoms you were, and after trying all the suggestions, the only one that worked was your own: creating a new project and copying all the contents across.
Then I noticed that I couldn't manually delete my Bin directory, and I realised that MyApp.vshost.exe had been running in the background this whole time, preventing itself from being overwritten by Visual Studio 2012. Not sure how it still worked from VS2010 though :/
In summary, the solution that worked for me: Kill the process, try again.
In other words, have you tried turning it off and on again?
