JVN jailbroke his iPhone 4 last year.
I thought about doing it to mine; but I subscribe to the philosophy that it's not a matter of IF something will be hacked, but WHEN it will be hacked. For something as personal as a phone, I'd rather stay in the thorny garden of Apple-y bliss (for the time being). When I swap phones, I'll probably jailbreak it just so I can get Java on it and use it as a dev platform for displays on a handheld device.