I bought one of those ebay sim unlock codes today, but before applying it I dd'ed partitions 1-33 just to see what changes. Doing a diff between the partition dump before and after the unlock I see partitions 3, 24, 25, 26, and 33 have changed. The process itself involved entering the code after inserting a non AT&T sim while the phone was booted. If I remember right it took a little while after the code was inserted and then wanted to reboot.
Do you guys think that it unlocks partition mmcblk0p3 while the phone is booted or that it somehow queues it for hboot to do on the next reboot? If its while its booted we could sneak a change to 0x8400 in while it's unlocked doing it's thing.
If anyone is curious the changes to mmcblk0p3 before and after the sim unlock is to locations 0xc25-0xc28 and 0xAC01-0xAE00