It looks like we will see an iPhone 4 jailbreak in the near future. George Hotz, the creator of blackra1n (a software that will jailbreak any 3.1.2. device) posted a snapshot on his blog of an iPhone 4 running Cydia. He might not release the software though:
The real reason behind no release isn’t technical. It’s just that it will never stop, after blackra1n, people demand unlock, after blacksn0w, people demand untethered. I miss the days when jailbreaking and unlocking were difficult, it attracted a much higher caliber crowd.
Meh [iphonejtag.blogspot.com]