Sequence of events:
2000 - SBC and BellSouth jointly create a company called "Cingular Wireless".
2002 - AT&T spins off a company called "AT&T Wireless".
2004 - Cingular buys out AT&T Wireless and merges it completely into Cingular.
2005 - SBC buys out AT&T, and rebrands themselves as "AT&T". So at this point Cingular is jointly owned by AT&T and BellSouth.
2006 - AT&T buys out BellSouth. Cingular is now wholly owned by AT&T.
2007 - Given that, Cingular rebrands itself as AT&T (legally "AT&T Mobility").