Looks like it did not take much time for the iPhone 4 to be jailbroken and the guy behind the Spirit jailbreak for iPhone 3.1.3 has successfully been able to run a Userland Jailbreak for iOS 4 on iPhone 4.
A recent update from @MuscleNerd, one of the guys behind the Dev team who develop iPhone jailbreaks recently posted a picture which shows a successful Userland Jailbreak on the iPhone 4 by @comex who developed Spirit.
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.
This would mean that the iPhone 4 jailbreak is just around the corner, however, it might be delayed because of an impending iOS 4.0.1 update coming on Monday to fix the Antennae issue with the newer devices.