Looks like the iPhone 4 has already gotten the jailbreak treatment. There are images floating around showing a proper jailbreak using a new version of the Spirit tool which should be publicly released next week. Updated.
Update: Uh oh. Apparently we might not see this jailbreak tool all that soon after all:
Well there is now little twist to it, another iPhone hacker named p0sixninja has confirmed that [Spirit] can't be released in its current shape because it contains some of Apple's proprietary code.
Guess we'll have to wait a bit longer. [Redmond Pie]