Apple’s choice of AT&T as the exclusive US carrier for the iPad is less a reflection of the AT&T network and more a recognition of the strength of GSM internationally. Apple gains by simplifying production by supporting 3G GSM network topologies. If Apple doesn’t want to produce a custom iPad for the US market it’s choice is limited to one carrier – AT&T. Verizon and Sprint use CDMA and PCS technologies and T-Mobile uses a different GSM spectrum.
Until Apple gets a dual CDMA/GSM chipset, or the USA goes with LTE across the board AT&T is realistically the only game in town for Apple.