Not to exacerbate the fears of anyone, but the issue with putting a child's name on clothing isn't so much a problem when a parent is within yelling distance, as it is when the parent is not. A parent screaming out the name "Megan!" (that's my daughter's name) doesn't give the predator any idea who Megan is, until the child actually goes to the parent, where they are safe. Like all criminals, these people are looking for the easiest opportunity and they aren't going after a kid who's ten or twenty feet away from a parent, or one who comes when called, they're going after a kid who is isolated or looks lost, etc. That's when a smiling stranger walking up and saying "Hi Megan!" may make a difference.
Having said that, I don't think this is an issue at Disney World and any one who wants to get personalized clothing should do it--it's fun. And I agree with what other posters have said--stranger abductions are rare, and we should be more concerned about the people we know than random strangers hurting our children. But I don't think the concerns with personalized clothing should be "pooh-poohed" (no offense, Pooh) as irrelevant.
Also, it's a good idea to discuss before your trip what your child should do if they get lost or separated from you, and who it is safe to get help from. When you arrive, a quick review at the park would simply be asking the child to point out for you who he/she would go to if they were lost, and so they know it isn't ok to go with that smiling stranger.
Karen