They revealed this in a tweet earlier today, sadly only the Windows version is up at the moment, however there is a Mac version being developed which will be coming soon.
You can download the Greenpois0n jailbreak here, the download itself is just 347kb in size and contains one exe, you can see a screenshot from the app at the end of this post.
The Greenpois0n jailbreak works with iPhone 4, iPhone 3G S, iPod touch (4th and 3rd Gen) and iPad, if you are using Greenpois0n on your iPhone 4 running iOS 4.1 you must use TinyUmbrella (here) to preserve your baseband.
Some early users suggest that Greenpois0n does not work, therefore we would appreciate your feedback in the comments section below.
Did Greenpois0n work for you? (Please state what device you was using and what Windows version you are running).