iPhone 4 was released on June 24 and a jailbreak for iPhone 4 did not take much time. iPhone 4 jailbreak was done by the person who also developed the Spirit jailbreak for iPhone 3.1.3. The iPhone 4 jailbreak is Userland Jailbreak for iOS 4 on iPhone 4.
An iPhone Dev team member @MuscleNerd recently posted an update congratulating @comex for developing another userland Jailbreak. Comex is the same person who developed the easy to use Spirit jailbreak for iPhone.
If you do not know, a userland jailbreak exploits the software or firmware on the phone and the exploit can be executed through a web browser by visiting a website, however, these kind of exploits can be patched out using firmware upgrades.
On the other hand, normal jailbreaks is usually at boot level and cannot be patched using a firmware upgrade and will require an hardware upgrade, which usually happens in newer versions of the device only.
Considering that an impending iOS 4.0.1 update coming on Monday to fix the Antennae issue with the newer devices do not expect a jailbreak for iPhone 4 to be released before that. We will keep you updated when a iPhone 4 jailbreak becomes available.