I got the iPhone 4 when the day it came out, locked AT&T. But I unlocked it to use with T-Mobile by using the software unlock from iPhone-Dev Team–Thanks guys!
When Apple released new firmwares (from 4.1 to 4.3.1) with new basebands (02.10/03.10/04.10), iPhone-Dev Team is still “in process” to unlock the new basebands.
Recently, Gevey (the company that offered the SIM unlocks for iPhone 3G iOS 2x, 3x) has announced the SIM unlock for iPhone 4, but the way it uses to unlock is illegal in the US because it requires to dial 112 and hangup–the emergency number in some countries, and you have to do each time when the device reboots.
On T-Mobile US, if you dial 112, it will automatically connect to 911, which you do not want to dial that number and hangup on them–you will get troubles.