The official iPhone 4 jailbreak is now finally available by Comex. Jailbreak 2.0 is the most easiest jailbreak method to date, all you need to do is visit the website Jailbreakme.com right on your iPhone using Safari, and just slide to to jailbreak! It's that easy. You should always back up and sync with iTunes before doing this. Jailbreaking is completely legal in the US. This website will also jailbreak your iPad, iPhone 3GS or iPhone 3G.
Update: This could brick your device. But there are reports of this working flawlessly.
However, this Jailbreak does kill Facetime and MMS. So DO NOT Jailbreak your iDevice this way if these features are important to you.