Eh it will be a nice battle...and the EVO surely helps Sprint. BUT there are still masses of iPhone users for AT&T, and the EVO isnt enough alone to overtake AT&T.
Now, if the rumors are true...and from Jobs own quote, that the iPhone could end up on 2 US Carriers....
Even if the other carrier isnt Sprint, it will destroy AT&T's market enormously....that alone could bring AT&T down to 3rd place.
I don't want to see iPhone on Sprint, and I don't think Apple will either. Can't see Jobs and the Apple board explaining how iPhone on Sprint at much better plan pricing (assuming Sprint stays as the value provider) while leaving it at high-cost ATT. Also don't see Sprint being able to give the concessions Apple will demand because Sprint doesn't have the $ or subscriber base to absorb the upfront losses.