It's a couple days later than we thought, but worth the wait; if you're running iOS 4.1, the GreenPois0n tool has been set loose for your jailbreaking pleasure. It's a Windows-only release for now—something they're hoping to correct "soon"—and supports iPad, iPhone 4, iPhone 3GS, and the third and fourth-gen iPod…
The just released limera1n promises to jailbreak iOS 4.0-4.1 on iPhone 4, iPhone 3GS, iPad, iPod Touch 4G, and iPod Touch 3G. It's Windows only for now, with Mac & Linux support coming soon.
We've seen videos of iPhone 4.0 Beta being jailbroken, but we thought we'd have to wait for quite a while until the jailbreak tool is released. Well, surprise! It's already available for iPhone 3G devices.