First, I don't think asking someone what you are doing in the neighborhood is a *nice* question to ask, necessarily. And I would bet it wasn't asked particularly nicely in this case. Certainly different than just saying hi, how're you doing? Or, to take suzi's classic example, can you help me find my puppy?
Second, what I would tell my little kid v. my teenaged son might be quite different. But I don't have a teenaged son, so I don't know that yet. What I do know, is that he will be unlikely to listen to me no matter what I say lol
All I can say is that you can't prepare your kids for every eventuality, so all you can do is teach them to use good judgment in the particular circumstance, and provide them with examples of what they might do if such and such happens. Honestly, and to your point, I don't think the precise scenario of this case is something that any parent previously would have thought to provide specific advice about. I just think that, given what is undisputed (at this point) about the exchange between Trayvon and George, that the best thing to do may have been for Trayvon just to have identified himself, and also that the argument that he was in legit fear of a random psychopath doesn't make too much sense to me under the circumstances.
jmo