Use your own ATT SIM with new jailbreak 1.1.2
I just got my iphone 2 days ago, so excuse me if this information was obvious/useless, but I think many other people have also been having trouble using their own SIM (ATT) on the iphones with the new baseband/modem firmware.
I downgraded to 1.1.1 from an OTB 1.1.2, jailbroke, got oktoprep, then upgraded back to 1.1.2. Ran the new 1.1.2 jailbreak, and put in my ATT SIM (that I just got 2 weeks ago, it was NOT meant for an iphone), and it works like a charm. Phone calls working great!
Thanks to everyone that made those awesome guides to do all I mentioned above :)