Since Jailbreaking iPhones now is also legal in the USA
, the website 'jailbreakme.com' has become pretty popular: You only need to visit that website, follow the instructions on there and your iPhone is jail-broken. This blog post by f-secure
shows how it works: Jailbreakme.com is simply using a PDF exploit. That's both interesting and horrific: If a website is able to jailbreak your phone, it can also do much more other, more evil things with it.
Great security, Apple.