Sprint only has their crappy proprietary 4G in a few cities. Check this coverage map and try to find where. I just did, and while I didn't look to hard to find it, its not even discernable on the map. Who wants their Wimax anyhow when EVERY other carrier in the U.S. is going to be using LTE as a standard. Sprint is just making it harder on themselves to make profit by going off on their own.
From what I've been hearing Verizon is getting the iPhone next year as well. So AT&T won't have much of an advantage over them any longer.
EDIT: Also for Verizon customers with Smartphones, sometime soon the term fee for the above mentioned phones is going to $350 on November 15. Guess that's why Verizon is always so highly ranked in how they take care of their customers.