As of January 26th, it is now illegal for you to unlock your smartphone if you want to use it on another network. Carrier unlocking has been legal in the U.S. for years, but in October the Library of Congress ruled that unauthorized unlocking is a crime.
The Obama Administration has already voiced its opinion that citizens should be allowed to unlock their smartphones without risking criminal penalties, and a senator from Oregon just introduced a bill that would making unlocking legal again.