|Apple has reviewed and updated a Knowledge Base article on the potential hazards of jailbreaking iPhones, following the release on Monday of the evasi0n jailbreak for iOS 6.x. While jailbreaking is not illegal, issues with the jailbreak (and previous jailbreaks for earlier iOS versions) have been reported, and Apple is within its rights to refuse service on a jailbroken phone. Minor issues have been reported with the evasi0n jailbreak, and the article has likely been updated simply as a reminder.
Previous jailbreaking solutions have been known to cause -- or at least be associated with -- increased battery drain, system instability, disruption of some services and more dropped calls. As jailbreaks are based on security flaws in the OS, others may find and exploit the same vulnerability to introduce malware that would otherwise not be successful. The use of unofficial apps also poses the risk of compromising personal information or running "zombie" processes that attack networks.
That said, jailbreakers generally do so on their devices simply to run apps that allow further customization or tweaks that Apple's iOS doesn't presently allow. A list of "top" unofficial apps looks mostly like a wish list for the next major revision of iOS, along with a few apps that allow things like unofficial "data tethering" where one's carrier forbids it. While battery drain and some instability are factors in jailbreaking, most problems are minor and the dangers Apple warns about in its article are more potential dangers rather than common issues.
The new evasi0n jailbreak for iOS 6.x devices has had a few issues reported, including the inability to access a user's "Purchased" history on the iTunes Store, a prolonged boot time and the disabling of the Weather app. The team behind the jailbreak have promised to work on the issues, and have seen high demand for the jailbreak -- stats from the unofficial app storefront Cydia of 1.7 million downloads in the past two days.
The Apple article specifically says that jailbreaking (which it calls "unauthorized modifications to iOS") can cause instability and risk the loss of data; creates the potential for introducing viruses and malware or increase the risk of hacking attacks, both common problems on the Android platform; accelerate battery drain (though its not clear if the jailbreak causes this, or the unofficial software jailbreakers generally load on their devices); less reliability in voice and data connections, and an inability to apply future software updates, which may contain critical security or bug fixes (but also close the vulnerability that made jailbreaking possible). It also mentions the possibility of "bricking" the iOS device, rendering it inoperable and sometimes unrecoverable.
Jailbreaking is generally only done by a tiny percentage of the overall iPhone user base, but Apple clearly views the procedure as a threat to both the security of the device as well as the quality of the experience, and are thus strongly discouraging of the hack. The updating may signal that Apple service people may begin refusing to service devices that have been jailbroken on the premise that the jailbreak may have caused the issue the customer is complaining about.