Starting February 11th, all four major US carriers will allow you to sim unlock your phone. The ban on phone unlocking was lifted last August by congress, and in the wake of this repeal the carriers are releasing their official unlocking policies.
Individual carrier unlocking policies:
Each company’s policies differ, but for the most part if your phone is paid off or your contract term is completed then you can request for your phone to be unlocked. Android Police wrote a very thorough post on the subject last week that you should check out if you want more info.