According to Macsimum News:

“The iPhone is one of the most anticipated handsets ever in the wireless industry, and we want to make sure that every drop of equity from the iPhone accrues to the AT&T brand,” AT&T spokesman Michael Coe told Reuters. “We want to be as far along as possible with our rebranding in advance of the launch of the iPhone.”

The article begins by stating, "AT&T plans to speed up its re-branding campaign at Cingular wireless stores, seeking to raise AT&T’s profile ahead of the launch of the iPhone next month."