This is driving me crazy. Ever since I installed ReSharper 4, F12 no longer seems to work. If you look at the all the ReSharper short cuts in the Goto sub menu Declar
ReSharper added a crap 'good' new thing that they now call a feature: Smart Go To Declaration. As dumb as it may sound, it comes enabled by default: crap 'good'.
The feature is described on the link provided from here but honestly at least it should've keep it disabled, not reinvent the wheel. It also tells you how to disable that crap 'good' feature.