This. Some people prefer knowing they're getting bona fide factory-unlocked or at&t authorized-unlocked iPhones to eliminate the possibility of reversing the inside job done by fusion_unlock, gevey IMEI unlocking, etc. as a lot of them have the disclaimer they will re-lock the iPhone IMEI of any returns or illegitimate purchases.
There has not been one documented case of them (AT&T, VZW, UnlockFusion, etc..) re-locking a phone once it has received the "Your phone has been unlocked" message in iTunes.
There was a claimed case on another forum, but it was later discovered that the phone had been unlocked via Jailbreak, and that the owner had updated (it was a 4S, not a 5 anyway...).
Again, I will offer a soda to anyone who can prove there was ever a case of a re-lock after ANY of the "shady" unlockers unlocked the phone.
There isn't. Just a lot of scare tactics because for whatever reason people want to propogate the myth that a factory unlocked phone is somehow different or better. It's not, and it isn't. They both share the same odds of being locked under normal circumstances: 0.
If it is reported as lost/stolen the IMEI can be blacklisted, but that is COMPLETELY different than re-locking.
After it's unlocked and in Apple's database (i.e. not via jailbreak), it's unlocked, and as of today, will remain that way indefinitely.
They could change that in the future, but it would be a COLLOSAL undertaking to figure out which were which since they both use the exact same means to unlock the phone (legit AT&T and UF).
They also could shut the unlockers out (in the future).
Still wouldn't affect unlocked phones already in the marketplace. AT&T (as an example) would be creating a nightmare for itself if it started relocking the presumably thousands of phones that have been bought, sold, and probably re-sold again (as unlocked), just because (even if they could).