I think they're still on Sprint, but I'm only ~70% on that.

Apple doesn't need AT&T anymore - the original exclusivity was because Apple didn't want any carrier interference (as carriers love to add their logos to handsets, install crapware into the firmware, lock-down settings and features, etc) - none of the carriers would agree except for AT&T, who demanded an exclusivity period to compensate for the risk of loss-of-revenue from evil things like only allowing photographs to be sent by MMS.