Is it Apple that maintains this list of unlocked IMEI's or AT&T? I know if you take your phone into Apple and ask them to check--they can see your iPhone is unlocked. However, one contacts AT&T to unlock your iPhone when the contract is up.
The list for all iPhone and iPad carrier locks is kept solely by Apple. No other company or carrier is able to add or remove a device without going through Apple. Be it a rouge Apple employee or legitimate channels.