Eating dropped pieces of food is fairly common in our house (depending on the type of food). If Asher fumbles his barely eaten burrito on the floor at dinnertime, we're more than likely say to him, "pick it up and eat it" in deference to the famous 5-second rule (also out of a desire to avert a tantrum and produce an alternative meal). I just read this NYT article, The Five-Second Rule Explored, or How Dirty Is That Bologna? and the answer is quick retrieval does mean fewer bacteria, but you should also consider which surface it touched.
Clemson University did several tests on floor to food contamination with E.coli and salmonella, and if the surfaces were contaminated, within 5 seconds food became contaminated. It only takes 10 salmonella and fewer than 100 E. coli to contaminate food. Those pesky salmonella can live up to 28 days. Fun fact: carpet retains tens of thousands more bacteria than wood or tile. Ewww gross.