Starting today, select U.S. carriers will unlock phones as per a policy change in the Consumer Code for Wireless Service. Note that these carriers will not unlock a phone until the contract has ended, and those eligible will be notified. (All deployed military personnel are eligible.)

More details:

The carriers and their policies:
-- AT&T Wireless
-- Verizon Wireless
-- T-Mobile
-- Sprint
-- U.S. Cellular