Just like Limera1n, GreenPois0n has hit the jailbreak arena with a surprise today. I am not sure why Chronic Dev-Team has released GreenPois0n tool when Limera1n was already out. Chronic Dev-Team used the same bootrom exploit which was used in Limera1n, hence saving the SHAtter exploit for the next-gen iOS devices.
I jailbreaked my iPod Touch 4G and iPad with Limera1n earlier. Today i re-jailbreaked both of them with GreenPois0n and found Limera1n much more stable and easy to use than GreenPois0n. GreenPois0n failed to detect my iPod Touch in DFU mode number of times while Limera1n detected it in one go.
I would recommend you to use Limera1n instead of GreenPois0n tool as it is more quick, stable and easy to use. Just incase if Limera1n is not working for you due to any error or compatibility issue, use can use step-by-step guide here to jailbreak iOS 4.1 on iPod Touch 4G, 3G and iOS 3.2.2 on iPad.
Update1: How to Jailbreak iOS 4.1 on iPhone 4 & iPhone 3GS with GreenPois0n Tool?
Update2: Jailbreak iOS 4.1 on iPhone, iPod Touch, Apple TV & iPad with PwnageTool 4.1
Update3: Jailbreak iOS 4.1 on iPhone 4/3GS/3G, iPod Touch 4G/3G/2G & iPad with Redsn0w
Update4: Guide to Jailbreak iOS 4.1 & iOS 3.2.2 on Windows with Sn0wbreeze 2.1