In case you had any doubts, the "Five Second Rule" doesn't work, says Dr. Susan Rhem, an infectious disease specialist from the Cleveland Clinic:

A common superstition, the five-second rule states that food dropped on the ground will not be contaminated with bacteria if it is picked up within five seconds of being dropped (Wikipedia).

Comments from Twitter:

@alisha764: The "5 Second Rule" doesn't work: Food + Floor = Bacteria

Source:
http://feeds.feedburner.com/CasesBlog