Per a tweet by a member of the iPhone Dev Team, an all-device jailbreak (like Spirit and Blackra1n) is seems imminent for the iPhone 4:

The tweet goes as follows:

“MuscleNerd: @xfsasx jailbreak for all devices at 4.0 is already handled by upcoming @comex release so, good time for iphone4 unlock stuff”

A specific iPhone 4 jailbreak release date has yet to be announced, though an iOS 4 update is due within a few weeks”

If you’re in the mood to run some extra apps as well as void your iPhone’s warranty, have at it and it should be a good release.