Getting an unlocked phone to work on AT&T is easy as all you do is put in a sim. It's a bit harder on Verizon and sprint. Verizon and sprint only allow phones they approve to be active on their network. Hardware wise, an iPhone 4s can work on all the networks (no tmobile 3G) but Verizon and Sprint have to allow it first.
Right... I've unlocked GSM phones w/ tmo before by simply calling and asking nicely. Any chance this will happen with sprint some day?
On the other hand i guess I could say what is the point... I'm off contract, so I could drop out any time, switch to another carrier and sell the sprint phone to cover the switch +the first few months.