Starting today, carriers must unlock a phone that is paid off
Starting today, if you have paid off your phone (regardless if it was done via a subsidized contract or an installment plan), or own a pre-paid phone for a year, all four of the major U.S. mobile operators have to unlock your phone if you make such a request. In addition, the carriers are now required to tell you when you can unlock your phone. If you are U.S. military personnel, the new rules require a carrier to unlock your handset if you so desire.
It wasn't terribly long ago that unlocking your phone was against the law. But last year's Unlocking Consumer Choice and Wireless Competition Act changed all that. If you own a phone lock, stock and barrel, you have the power. Each carrier has their unlocking policies listed online, and requests to unlock a phone must be taken care of within two business days.
Once you unlock your phone, it can be used on any compatible network no matter who the carrier is. Yes, it's a brave new world out there.
source: arstechnica, AT&T, Verizon, Sprint, T-Mobile