The iPhone does not require the SIM it came with. However, if you change the SIM to one that has never been in an iPhone, it deactivates the device and requires reactivation via iTunes. My old unhacked phone worked fine for a friend of mine when he popped in his AT&T SIM, but only after plugging it into iTunes and adding an iPhone data plan to his account.

There isn't a software unlock for the 3G yet, and no estimate on when one will exist.