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Kai Ryssdal: The more tech-savvy among you may have known this, but it was news to me when I read it this morning -- that there's actually a word for when cell phone users hack into the software that runs their devices. "Jailbreaking" it's called. It's done mainly to skirt the rules and let people use their own free ringtones or applications. Apple, for example, blocks all but officially authorized apps on its iPhone. Even just trying that has been, technically, a violation of federal law. But no more, thanks to some new rules out from the Library of Congress.

Our senior business correspondent Bob Moon explains.

Bob Moon: What if we lived in a world where carmakers slammed the hood closed...

Hood squeaks and slams shut

...and then welded it shut.

Sound of welder

That's the argument the Electronic Frontier Foundation made for letting users unlock their cell phones legally. EFF's Jennifer Granick says carmakers can't dictate certain spark plugs, and cell phone companies shouldn't limit your programs or ringtone choices.

Jennifer Granick: The idea here, I think, is you bought it, you own it, it's your device, and you should be able to be the one to make that device most useful for you.

Theoretically, it's also now legal to switch devices to new carriers. But Forrester analyst Charles Golvin says "legal" isn't the same as "possible." iPhones, for example, can technically run on T-Mobile's system, but just some frequencies.

Charles Golvin: The speed of the data would be very, very slow compared to what's on AT&T. And of course, the technology is completely incompatible with Sprint or Verizon.

Golvin says users are likely to be more comfortable with the reliability of authorized apps, so he sees little impact for companies that lock users to their own offerings. And unlocking any cell phone can mean trusting unknown hackers. You could still void your warranty, and the EFF's Jennifer Granick concedes Apple and other companies won't give up control easily.

Granick: There has been kind of a technological arms race with Apple disabling the "jailbreaks," and then researchers figuring out how to re-jailbreak it, and then consumers doing that, and then Apple issuing updates that break the jailbreaking, and then the whole cycle repeats itself. And nothing about this is going to stop that.

Granick says the ruling legalizes consumer tinkering, but won't tie the hands of cell providers.

I'm Bob Moon for Marketplace.