Sprint might do well by offering an unlimited data plan to their iPhone users, at least for a reasonably extended promotional period. That, along with a guaranteed grandfathering deal could get them off to a nice start. They are still one of the best, and getting the iPhone could put them right back up there. I don't know what they have been doing with their WiMax and 4G plans lately; but, they have been working on those aspects for a while now. Perhaps they might hint at sooner access to higher speeds than the others...
Although I did see an article about how AT&T and Verizon were unlikely to lose many customers to Sprint, it may be a company worth watching fairly closely.