Just so people realize, though, "The new at&t" is not the same as the old "AT&T Wireless".
AT&T Wireless was created by the old "AT&T", then spun off as a 100% independent company. It just retained the AT&T name.
Cingular was formed as a joint venture by SBC and BellSouth. Cingular bought AT&T Wireless, then essentially just closed it, transitioning all former AT&T Wireless customers over to Cingular, rather than actually merging the two companies. Years later, SBC bought AT&T, (no, it was not a merger, SBC is definitely the 'parent' company in the deal,) and then renamed itself at&t, bringing 50% of Cingular's ownership under the "at&t" name, even though it really is a different company. This new at&t then bought BellSouth, bringing Cingular 100% under "at&t" ownership.
(Of course, if you want to get *REALLY* complicated, SBC and BellSouth were both formed by the 1984 breakup of AT&T in the first place... So it's all the same 'family' all along.)
And as for the 'Cingular' on the display of the iPhone, most cell phones get their network name from the network itself. So, for example, when Cingular bought AT&T Wireless, cell phones started saying "Cingular" instead of "AT&T Wireless" on their displays, with no software update needed. Likewise, if you have an unlocked GSM phone, you see either "Cingular" or "T-Mobile", whichever you happen to be connected to. (I have an unlocked T-Mobile phone, yet when I roam into an area with a strong Cingular signal, and no T-Mobile signal, my phone's display changes to read "Cingular".)