According to Apple Insider (http://www.appleinsider.com/articles/11/01/11/non_exclusive_apple_verizon_deal_could_allow_iphone_on_sprint_t_mobile.html), because Apple has a non-restrictive agreement with Verizon, it is possible that similar agreements could be struck with Sprint and T-Mobile. The Verizon iPhone uses the CDMA network, which is popular all around the world, making this a potential boon for an already successful Apple.
Verizon iPhone can act as mobile wi-fi hotspot
Unlike AT&T iPhones, Verizon iPhones can act as mobile WiFi hotspots like Sprint’s EVO 4G. And if Sprint does that now, bets are good it will work on the Sprint iPhone as well.