FWIW - AT&T is now in SD. (grew up there, and still have family in the state).
I was in MN back then (when I took the trip). ATT was expanding then in north mid-west. Even in MN, their coverage improved between 2007-2010.
I was driving through I-90 which cuts across SD and even stayed overnight in Rapid City. I think I had good signal, which I must admit was surprising on ATT, though, I wasn't aware it was from their partner n/w backs then.
I found this incident funny, 'cos this business had the sense to know how telecoms worked, knew the law and exploited the loophole to make money :-). Sometimes, I think its just connecting the dots as Jobs said.
Verizon purchased Alltel, but was required by the FCC to sell off 105 of the markets, which is where ATT stepped in and bought a bunch of those markets.
FWIW, my Alltel account was switched over to Verizon in 2009.