The Library of Congress has approved a copyright exemption that will allow consumers to legally unlock their mobile phone to change to a competing carrier. The LoC decided that locked handsets don't qualify under the Digital Millennium Copyright Act (DMCA). It was found that the locks weren't in place to protect the cell phone developer, but instead third party interests (carrier subsidies). The ruling does not outlaw locking handsets, but more simply says that it's not illegal to unlock a handset. There has been at least one lawsuit from a carrier against a customer who unlocked their handset.
The ruling takes effect today and expires in three years. Full documentation of this exemption and five others also approved is available in a PDF.