It sounds ridiculous, but beginning next week, it will officially be illegal in the United States to unlock a smartphone without your wireless carrier's permission.
Last October, the Librarian of Congress determined that certain actions involving mobile phones were illegal under the Digital Millenium Copyright Act. The rules were revised to say that while it is legal to jailbreak smartphones, it is illegal to jailbreak tablets and illegal to unlock phones without permission from your wireless provider. A 90-day window was established to allow consumers the chance to buy a smartphone and unlock it, but that window will be closing this weekend on January 26.
In the U.S., most carriers lock their phones to stop consumers from running them on competitors' networks. Next week, it will become illegal for U.S. consumers to unlock their carrier-locked devices without permission.
Some smartphones, including Verizon's iPhone 5 and the Nexus 4, are already unlocked at point of purchase.