You will have heard that the US Copyright Office has lifted the temporary ruling under which you were allowed to unlock your phone. EFF explains in detail what this ruling means (it's not what you think -- and in some ways, it's worse):

First, the good news. The legal shield for jailbreaking and rooting your phone remains up - it'll protect us at least through 2015. The shield for unlocking your phone is down, but carriers probably aren't going to start suing customers en masse, RIAA-style. And the Copyright Office's decision, contrary to what some sensational headlines have said, doesn't necessarily make unlocking illegal.

Unlocking is in a legal grey area under the DMCA. The law was supposed to protect creative works, but it's often been misused by electronics makers to block competition and kill markets for used goods. The courts have pushed back, ruling that the DMCA doesn't protect digital locks that keep digital devices from talking to each other when creative work isn't involved. And no creative work is involved here: Wireless carriers aren't worried about "piracy" of the software on their phones, they're worried about people reselling subsidized phones at a profit. So if the matter ever reached a court, it might well decide that the DMCA does not forbid unlocking a phone.

Now, the bad news. While we don’t expect mass lawsuits anytime soon, the threat still looms. More likely, wireless carriers, or even federal prosecutors, will be emboldened to sue not individuals, but rather businesses that unlock and resell phones. If a court rules in favor of the carriers, penalties can be stiff - up to $2,500 per unlocked phone in a civil suit, and $500,000 or five years in prison in a criminal case where the unlocking is done for "commercial advantage." And this could happen even for phones that are no longer under contract. So we're really not free to do as we want with devices that we own.

All that said, if you were convicted, the maximum penalty under the law for unlocking your phone is now greater than the maximum penalty for turning it into an IED.

Is It Illegal To Unlock a Phone? The Situation is Better - and Worse - Than You Think