"Jailbreaking" your iPhone (that is, modifying a phone so it runs apps that aren't approved by Apple) is now legal. The U.S. Copyright Office, which is part of the Library of Congress, approved it last week. 

For the vast majority it means anything. But for tech savvy jailbreakers and app developers (who don't care voiding Apple's warranty) this is extremely exciting. 

For example, one unapproved application enables tethering (the ability to share the iPhone's Internet connection with a computer, something for which iPhone owners are supposed to pay AT&T an extra $20 a month. For me, this is particularly useful when you are abroad and you only have an international data plan on your smartphone but not on your laptop. 

Browse the internet and you will find an explosion of forums, blogs and sites (as well as stores selling unapproved applications) guiding users through the jailbreaking process, like Hack that Phone.

There's a whole world of possibilities for what you can do with your iPhone.

But remember whenever Apple introduces a new version of its mobile operating system, relocking its phones, you need to jailbreak it again.

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In addition to this decision, the Library of Congress also granted an exception to artists who remix copy-protected video content for noncommercial work. Also it renews its approval for cellphone owners to "unlock" their phones or lift controls that restrict use to one wireless carrier.