According to the article it's on LTE, so that means AT&T or Verizon. I can see why they're doing it as it looks like WiMax doesn't have a great future ahead of it, but I both those carriers have abysmal track records for customer abuse (as opposed to merely bad.)
4G networks will have to fall back on 3G networks sometimes, which is one of the reasons why AT&T is still building out its backhaul for its 3G network. Though I agree, it would be nice to have interoperable phones