From this Saturday, it will become illegal to unlock a smartphone in the US without permission from the carrier it was locked by. This follows changes in the DMCA (Digital Millenium Copyright Act) made in late October of last year. The changes only affect new phones, however, and will not impact used phones.
The rule is intended to prevent users from purchasing a discount phone with the intention of unlocking it. Phones that have been purchased unlocked will also be exempt from this ruling. The application of this ruling will most likely be passed to the mobile carrier, who could then track those who breach the ruling with the backing of the DMCA.