Apple has posted an article (http://support.apple.com/kb/HT3743) warning against the jailbreaking of an iPhone. The article is about adverse issues experienced by customers who have made unauthorized modifications to the iPhone OS (this hacking process is often called “jailbreaking”).
Apple says that issues that have been encountered include instability, disruption of services, and compromised security. “As designed by Apple, the iPhone OS ensures that the iPhone, iPad, and iPod touch operate reliably. Some customers have not understood the risks of installing software that makes unauthorized modifications to the iPhone OS (‘jailbreaking’) on their iPhone, iPad, or iPod touch,” Apple says. “Customers who have installed software that makes these modifications have encountered numerous problems in the operation of their hacked iPhone, iPad, or iPod touch.”