TELECOM Digest Editor noted in a response:
Cingular was an LLC owned 60/40 by SBC and Bellsouth. SBC was the company formerly known as Southwestern Bell, but they changed the name to their stock ticker symbol SBC as they bought and absorbed PacTel, Ameritech, SNET, and some other stuff I don't remember.
Quite a while ago, AT&T bought Craig McCaw's cellular properties and rebranded them as AT&T. In keeping with AT&T's londstanding policy of slowly bleeding itself to death, since the wireless properties showed some promise of being profitable, AT&T spun them off as a separate company, AT&T Wireless (ATTWS).
A year or two after the spinoff, ATTWS was bought and merged into Cingular, with all of the former AT&T stuff re-rebranded as Cingular. The integration from that merger is mostly complete, although there's still a few places on Cingular's web site where it says to click here if you're a former ATTWS customer.
Then after that, SBC bought the tired husk that remained of AT&T, and decided to confuse us all by calling the merged company AT&T, even though it is a whole lot of SBC and not much AT&T.
Last year, SBC, now AT&T, bought Bellsouth and has been merging it into the new AT&T. This means that Cingular is now entirely owned by the new AT&T. Having just finished the expensive process of re-rebranding all of the former ATTWS properties from AT&T to Cingular, now they are starting the even more expensive process of re-re-rebranding Cingular back to AT&T.