The U.S. Copyright Office slapped Apple in the face by declaring iPhone jailbreaking legal, but that won't change anything for iPhone users.
Every three years, the copyright office considers exemptions to the Digital Millennium Copyright Act, which forbids people from breaking copyright control mechanisms. Apple fought to ensure jailbreaking's inclusion in the DMCA, presenting the practice as dangerous. Apple's most sensational claim was that jailbreaking could knock out network transmission towers, but the company also listed App Store piracy and security risks among its concerns.
Jailbreaking is the process of freeing an iPhone from Apple's software restrictions, opening up the phone to unauthorized apps and total customization. Hacks such as Spirit and Pwnage Tool make jailbreaking possible, but Apple has tried to thwart these measures with operating system updates and by adding features that were once only possible through jailbreaking, such as third-party multitasking and custom wallpapers.
The copyright office's ruling means that it's not illegal to jailbreak an iPhone. So why wouldn't you? The same reasons as before: The iPhone may be less stable depending on what unauthorized apps you install, has a slight chance of security breaches and may have difficulty upgrading in step with official iOS releases.
Also, your warranty is still void if Apple finds out, and the copyright office's decision won't make a difference. "Legal" doesn't mean "must be covered under warranty." Cracking open your laptop and tinkering with the innards is legal too, but don't expect a free repair if something breaks.
To be sure, there would have been consequences if the government went the opposite way and didn't grant a DMCA exemption. I doubt Apple would have called the cops on anyone who strolled into the Apple Store with a jailbroken phone, but the company could have possibly ordered DMCA takedowns of websites that offer jailbraking software, while using the risk of legal action to cause a chilling effect for hackers and people who were thinking about jailbreaking.
I'm glad people can still jailbreak their iPhones without those risks -- in some way it forces Apple to compete with the features of jailbroken phones -- but don't expect the process to become any easier as a result.