Look, we're gonna be up front about this. You probably won't want to do this iPhone hack -- it requires special equipment, it's a little difficult (to say the least), and it may not even work for data apps anyway. There, now that we've gotten that out of the way, let's talk about what's at hand.
So we saw something similar to this before, but Hackint0sh user Sassha apparently managed to "unlock" his iPhone and post a guide to using a SIM writer device, SilverCard, a SIM with accessible Ki number, and, of course, some some software so you can do the same. Basically, you have to specially reprogram your new SIM's IMSI to match your AT&T SIM, essentially tricking your iPhone into thinking it's using the real deal SIM when it's actually on a different network entirely. Then, after a few (read: many) more steps and a sprinkling of fairy dust your iPhone could be carrier-free -- but with no guarantees data will work (since it's not like Apple gives you a place to configure your EDGE connection). In other words, caveat emptor, use at your own risk, and heed word to the wise: just wait for the real unlocking-hacks to be released. The iPhone's pretty good, but it's not good enough to jump through these kinds of hoops for no certain outcome.