That was pretty funny...didn't get as big a reaction as I expected, though.
This prank reminds me of one I had seen played in the office I work in. Someone in the group downstairs had decorated a shoebox in a Hallowe'en motif and put a sign up that said "Hallowe'en candy - Help yourself!". Inside the box was a large rubber rat that was attached to the top of the box by a short string. When you opened the box, the rat would rise up with the lid as if it was coming out. The prankster set the box in the break room and boy, did the hilarity ensue!