Comex has released his jailbreak for all iOS 4 devices including the iPhone 4. He tweeted a link to JailbreakMe moments ago.
Once you have jailbroken your iPhone on iOS 4 you will be able to unlock it using UltraSn0w.
Congratulations to comex and the iPhone Dev-Team for making this happen.
Guides
How to Jailbreak Your iPhone Using JailbreakMe [4.0.0, 4.0.1]
How to Jailbreak Your iPod Touch Using JailbreakMe [4.0.0]
Jailbreak Released for All iOS 4 Devices Including the iPhone 4 + Guide
