It’s here and you’re probably jumping with joy. Greenpois0n is up and can be used to jailbreak iPhone 4, 3GS, iPod Touch 4G on iOS 4.1, and iPad on iOS 3.2.2, but there’s no unlock. If you rely on an unlock for iPhone, don’t update to iOS 4.1. If you already have, wait till November when Apple releases iOS 4.2 and then hopefully the iPhone Dev-Team would release the unlock. THIS ONLY WORKS FOR WINDOWS. You need a WI-FI or 3G connection for this to work (Cydia updates).

Follow this guide to jailbreak the iPhone 3GS and 4 on iOS 4.1:

STEP 1: Download Greenpois0n (link here)

STEP 2: Connect your iPhone to the computer via USB, power off your iPhone and launch Greenpois0n jailbreak tool.

STEP 3: Click “Prepare to jailbreak” (DFU).

STEP 4: There will be a series of on-screen instructions, so follow everything you’re told to do.

STEP 5: Now your iPhone can be jailbroken. All you have to do is click on “Jailbreak”.

STEP 6: Your iPhone is now jailbroken and you can see a “Loader” icon on your iPhone’s SpringBoard.

STEP 7: Press “Loader” icon and “Install Cydia”.

STEP 8: After Cydia is installed, a window will prompt you: “Do you want to remove Loader from your device”. Click “Remove”.

STEP 9: Through a Wi-FI or 3G connection, launch Cydia, and installation files would be downloaded and automatically installed.

Now you’re running an iPhone 3GS or 4 jailbroken on iOS 4.1.

[Thanks to iClarified for the last two screenshots]