I'm about to buy a used locked iPhone that I want to unlock for t-mobile using the software gui. Do I need to have a SIM card in order to do that? The person who is going to sell it wants to keep their SIM for a different phone and I don't want to sign up for service just yet.

Also-is it possible to get video out support on an unlocked iphone or is it only through the firmware update at this point? Thanks guys

if the phone is only 1.02 you can unlock it and use it with any sim. if the phone is 1.1.1 you wont be able to lock it at the moment. video out support is part of the 1.1.1 update

I understand that I can unlock it to use it with any sim-but I need to know if I need the sim for the unlocking process itself-sorry if i was unclear

you don't need any sim in the phone to unlock
follow a guide
carlos one is the best

http://hackint0sh.org/forum/showthread.php?t=6637