You probably can't find a website because of the liability involved. Anybody could put up information that any device was stolen. How is the operator of the website supposed to prove it one way or another.
I can't recall but does the original box have anything other than the serial number? If the phone takes a walk how do you get the IMEI outside of having to go pry it out of AT&T to post somewhere? If AT&T doesn't care enough to blacklist IMEI numbers then I don't suppose they're going to be all that cooperative.
Unless one location would be considered the end all, be all of locating a stolen IMEI then it should be the job of Apple or AT&T to maintain such a list since it inevitably comes back to them (for a legitimately activated phone). If it's a unlocked phone then it probably doesn't matter anyway since it's not going down any legitimate channels.