It is not long since the iOS 4 has been released (a Gold Master Release was made available for developers only yesterday), and it has already been Jailbroken for the iPhone 3GS (video below). Still, this is not a ‘touched up’ iOS 4 Jailbreak, so it has not been highly recommended for a non-expert iPhone user. Jailbreak at your own discretion. iPhone HD not out yet, so no Jailbreak for it yet.
Official Update (June 23, 2010): Official iOS 4 Jailbreak for iPhone 3GS, iPhone 3G and iPod Touch 2G from the iPhone Dev Team.
UPDATE: check out the iPhone 3G and iPod Touch 2G iOS 4 Jailbreak including instructions.
The new iPhone 3GS iOS 4 Jailbreak has been shown to work at RedmondPie, along with instructions on how to use a combination of Pwnage tools. They also mention that the iPhoneDev Team promised an updated and ready iOS 4 Jailbreak method within this month; so waiting is preferable.
If you decide to wait and would like to Jailbreak the previous iOS, then check out the various posts we provided at arriving at that goal:
– How To Jailbreak iPhone 3.1.3 Firmware with SnowBreeze 3.1.3
– How To Jailbreak Your iPad, iPad 3G and iPhone 3.1.3 with the Spirit Jailbreak