Unlocking only requires a jailbreak of the phone and from there it can be unlocked with a software. Even if unlcocked, it is impossible to use on the sprint network. The iPhone isn't CDMA compatible. It's built for gsm (sim card).
AT&T iPhones cannot be unlocked by code at the moment only by jailbreaking. Sprint will cannot activate a GSM device on their network. Cellunlock is a very reputable unlocker as are we.