The release of iOS 4.1 meant plethora of features for iPhone users; Game Center, Air Print, Air Tunes, the list goes on. With all the updates, Apple also included code that overrides jailbroken iPhones to return them to their normal, pre-jailbroken status. On October 12th the Chronic Dev Team released their newest option to jailbreak iOS devices, Greenpois0n. This jailbreak is currently only available for Windows, but supports iOS 4.1 on the iPhone 4, 3GS, 3rd and 4th Generation iPod Touch, and iPad.
Though if you're currently using an unlocking program like ultrasn0w so you can use your iPhone on T-Mobile's network, you might think twice before updating.
currently, ultrasn0w and other types of unlocking utilities do not work once your iOS 4.1 iPhone has been jailbroken. This means that if you do update to 4.1 and use T-Mobile, you will be out of luck... But at least you'll have Cydia!
Here is the link for your downloading consumption.