Mmm... letting my players play in a world full of zombies was one of my insane scenarios I intended to try.
The first was identical to the movie "Zombi" by G.Romero, a masterpiece. Using some device like the one presented in the Tiphareth's citadel in Metropolis sourcebook. (Where the PCs end in Evil Dead movie).
The other insane scenario had setting in the movie "The day after" letting the PCs deal with a total termonuclear war...