The BBC made a nice little video "testing" whether the "5 second rule" had any merit. Here's the video (not sure if outside UK readers will be able to see it?)
Basically it's not a good idea as the food samples dropped on the floor for 5 seconds all had a lot of nasty bugs on them after culturing them overnight on bacteria friendly plates.
Fortunately the "mum" points out that the cleanliness of your own floor is also an important factor.
There is one flaw with the experiment though as they don't have a true negative control - unless I'm misunderstanding their "0 second" test. Basically they need to put a sample of bread (or apple) that hasn't been dropped on the floor. It could well be that the culture results are from the food itself and not the floor. They could also do a floor imprint as well while they are at it. If there's as many cultures on the plate for the sample that never contacted the floor then the contamination is inherent in the food. In which case the 5 second rule still applies! There's still hope.
Still a commendable bit of everyday science on the BBC.