How important is Apple to AT&T's business?

AT&T’s strained, outdated network and spotty coverage has long been a sore point for US iPhone users, with many calling for Apple to bring its popular phone to other carriers, thus giving users more of a choice. The rumor mill has even posited that the AT&T/Apple divorce could be no more than a year away and a possible open relationship with Verizon Wireless happening well before that.