The usermanual and software all seem to imply that it's locked to Cingular. It came with an AT&T sim card. It wouldn't accept my sim card. Cingular can't activate it the AT&T card on my account. They can sell me a new sim card tomorrow -- by the time I got through charging it and working with tech support on the phone, the local retail offices were closed.
I've got it synced to Critter's laptop, and I've been playing around with the camera, since I can't use the phone features yet. It looks like I won't be able to set ring-tone per person, however. :( On the bright side, I've got photo caller-id on it. :)
On the down side, in the process of fooling around with sim cards, I managed to deactivate my current sim card, so I don't have a phone for a day.