CBS actually did a story on this very topic a few months ago, although the reporting they did wasn't very thorough. The reporter only pointed out the fact that putting personal information about a child on the back of a vehicle might make it easier to contact one of those kids at a sporting event they play in or an extracurricular practice they attend. However, they offered no statistics on how many times that's actually happened. My husband and I own a media production company that offers news bureau services, so we did a little research. We couldn't find a single case of foul play or abduction resulting from information taken off of the back of a vehicle in Lee, Collier or Charlotte counties. That's not to say it's never happened anywhere, but we think the chances are relatively slim.
I do agree with you that advertising your child's name, birthdate and photograph on the back of your car is probably not a great idea. (I wouldn't do it.) I'd be less concerned about abduction (only because my kids never go anywhere except school without me) and more concerned about someone using that information to steal identity. My 14 year old son is an honor student. His school provides me with a generic bumper sticker that I proudly display on the back of my SUV. Southwest Florida knows I have a really bright kid and my son knows I'm incredibly proud of him. That's all they need to know.