i have an unlocked iPhone 3gs that i just restored. however, it is asking for a sim card to continue the restore processs, but i don't have a AT&T sim card i only have a tmobile sim card. Any help on how to get around this?
Did you simply press the restore button in iTunes and restore to 4.3.5? Unfortunately, that's an irreversible process and it's not the proper way to restore/update an unlocked iPhone. There are several steps you have to take to ensure the phone remains unlockable and in order to hacktivate it (alternate way of activating). The latest 3GS baseband is not software unlockable with ultrasn0w on the 3GS.
You do have a couple of options for unlocking it again, but they both have drawbacks. You can use a GEVEY SIM (ultra model is the best) or you can install the iPad baseband and unlock with ultrasn0w. The GEVEY SIMs don't always work very well, may not be legal in certain countries and they are not free. Installing the iPad baseband voids the warranty, may break GPS, and causes non-custom firmware restores in iTunes to fail. You'll need to create and use custom firmware from that point on.