if t-mobile or sprint got the iphone it would absolutely crush not only Verizon (which ive had for years before switching to AT&T) but AT&T as well. Cheaper prices with very good coverage you cant beat, but T-Mo is lacking in its 3G areas thus far, although something like this would really help expand their user base.
I dont see it crushing AT&T or Verizon. Most people are happy with their AT&T iPhone in their area and T-Mo has its fair share of network coverage issues and is still GSM-based, which is part of the reason for many of the complaints over Verizon. Id est, the voice quality and soft v. hard handoffs. These issues jut carry over to T-Mo.
Still, having two carriers will help many that simply wont or cant move to AT&T from Verizon, but I think that Sprint will be the most hurt by the iPhone moving to T-Mo. The only caveat is that T-Mo may not offer the same rates for the iPhone as they do their other plans.