I don't know if anyone here actually uses the 5 second rule, but...

The five-second rule: It just may be true

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A pair of biology seniors at Connecticut College decided to test the oft-cited maxim that food that sits less than five seconds on a floor remains safe to eat.
They used a well-traveled section of the college cafeteria; apple slices "because they were free," and Skittles, because there's an assumption that dry food is less susceptible to contamination, said researcher Molly Goettsche. To simulate the real world, food was dropped off the edge of a cafeteria table. One pair of edibles sat on the floor for five seconds; others for 10, 30, 60 seconds, and, finally, for five minutes. They repeated the experiment under the supervision of a biology professor.

The results: No bacteria were found on any food left on the floor for up to half a minute. Apple slices and Skittles both got germy after a minute (although one Skittle took five minutes).

Goettsche, 23, who is heading for a career in biotech market research, said she would not eat from a floor despite their findings.

"Use your discretion," she advised. "I don't want to be responsible for changing the five-second rule to the 30-second rule."

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I generally don't use stuff that fell on the floor. It depends on how hungry I am and how long it is since Rochester sat on the floor with his bare anus. Anyway, I'm prertty sure there was a Mythbusters or similar which proved exactly the opposite. Stead might be able to confirm that for me as I beleive he's a fairly regular watcher of it.