Unlocking a phone to use it with another carrier will be illegal in America from midnight tonight unless the cellphone is already in your hands.

Freeing a phone from a particular network used to be allowed, but under last October's reinterpretation of the US Digital Millennium Copyright Act that right disappears from all handsets bought from Saturday. Jail-breaking devices - to allow the installation of third-party apps - will remain legal, only not on tablets.

The Librarian of Congress has the ability to grant exceptions to the draconian DMCA, and back in 2010 decided that unlocking should be allowed, but those rulings are reviewed every three years and in 2012 there was a change of heart: Americans were given three months to get used to the idea - three months which runs out tonight.

At issue is the ownership of the device as the new rules restrict the rights of the apparent owner to use the technology as they feel fit. It also makes little sense as we could soon see US courts being asked to decide where exactly the line falls between a big phone and a small tablet - American law does not acknowledge the existence of the phablet.

So circumventing the carrier lock on a cellphone will constitute a breach of the DMCA, as will installing Cydia on an iPad (but not an iPhone). Android users rarely have to circumvent blocks to install software, but will also have to avoid unlocking handsets.

Operators lock phones to give them enough time to recover the subsidy they pay when the contract is signed; typically a phone is free or rather cheap and the monthly connection bill makes up for it. Operators remain eternally optimistic that customers will send each other expensive multimedia text messages or download games from the operator's portal, all of which goes into calculating the subsidy and will be missed if the customer switches networks.

The answer is to not buy subsidised handsets, and it’s the availability of such phones that convinced the Librarian of Congress to change the rules. So if you want a subsidised handset you can unlock then best buy one today, otherwise you'll have to pay full price, or not be an American. ®