The 5-Second Rule.

Though it obviously depends on factors like how dirty the floor you're dropping the food on is, what kind of bacteria, what kind of food is dropped and so forth. Not to mention that just "a floor" won't be a proper analogue for all floors out there, the only way to really simulate it is to create a floor with an even amount of bacteria spread over the surface.

Mythbusters did a test similar to the students but used an analogue and found out that it doesn't really matter if it sits one second, five seconds or longer (within reasonable time range) when it comes to bacteria pick-up.

In Snopes they refer to another study with exacly the opposite conclusion:

http://www.snopes.com/food/tainted/dropped.asp

Saludos!

Mythbusters also studied this and came up with the opposite conclusion.

What if you're eating an apple and drop it right into a pile of dog crap?

When I worked at a country club as a chef and a trainee asked if we had a "5 second rule", I said "what is that?" He said, "you know, if you pick it up within 5 seconds you can still serve it." I told him, "no, we have a 'footprint rule', if you can't see footprints on it, go ahead and serve it."

Why do you have dog crap on your floor?
Clean it up, you pig.

What if you're eating a big sloppy piece of pizza and you drop it in front of the urinal trough in the men's room of Fenway Park?

If it were really safe, then you could also suck spilled liquids off the floor.

Whatever you do, just don't drop it on some ice.