AT&T is one of those companies people love to hate (like Microsoft). If they had just kept the name SBC when they bought the original AT&T and took it's name, they'd probably be significantly more loved (as much as a phone company can be). Instead most people like to exaggerate their issues and pile on cause it's fun. Human nature.
All of that said and bringing it around to the original topic, it's great to see a telecommunications company getting serious about a connected car and not just slapping a modem in the dash and sending you a bill (which is what they've done so far). Take a look at their actual press release:
http://about.att.com/newsroom/connected_car.html
The lab they built out in Atlanta actually looks like something i'd expect from Elon, so I'm guessing this had more to do with a aligning of vision then just a "we're cheaper" sales pitch. Now that I think of it, does Verizon even want into this space?
Many people seem to be unhappy with Tesla's choice of AT&T, but I don't know that they had any other good choices.
In the U.S., there are 4 major nationwide carriers:
(1) Verizon
(2) AT&T
(3) Sprint
(4) T-Mobile
The Model S was designed with 3G Connectivity in mind. If the design specifications were being finalized in the 2010 timeframe, LTE hadn't been around that long, so Tesla needed to choose a 3G Carrier.
Verizon, if I remember correctly, uses the EV-DO protocol for its 3G-class network. EV-DO is a CDMA2000 technology, and CDMA is not widely used outside of North America and Japan. It would make less sense to use CDMA2000 technology than GSM-based protocols like UMTS.
Sprint also uses a CDMA network, and bet big on WiMax. Using Sprint would have had the same problems as using Verizon, and perhaps worse. Sprint's network has been ailing, although Softbank's acquisition may turn things around if they can do a good job completing their LTE network.
T-Mobile uses a GSM-based network. However, the T-Mobile network is much spottier than the larger carriers, at least outside populated areas.
That leaves AT&T. They had a GSM-based 3G network, which was better developed at the start of this decade than T-Mobile's network. This was really Tesla's only good choice IMO.
Regional carriers and MVNO's were a possibility, but it's simpler on the business end to deal with one nationwide carrier than sew together agreements with a lot of smaller carriers, or go with an MVNO who depends on being able to buy network bandwidth from a big carrier.
What I think would be cool would be for future Tesla cars to offer a user-accessible SIM slot, and for the owner to choose their data provider.