For those of you who are nostalgic for older firmware, you will be pleased to hear that the iPhone Dev-Team has just released a solution that allows newer iOS devices like the iPhone 4S, iPad 2 and the new iPad to be downgraded to lower earlier firmware versions.
The fix is called RedSn0w 0.9.11b1 and allows for those who unwillingly or accidentally updated their devices to iOS 5.1 or iOS 5.1.1 to go back to their preferred versions of choice iOS 5.0.1. for an untethered Jailbreak. But users willing to get started will have to meet a set of requirements before they can proceed to the next level. You will have to make sure you have your SHSH blobs from iOS 5.0.1 saved. You will also not be relying on an unlock that will require a certain baseband version.
Provided you meet these two criteria you can move on and get your device downgraded. The first step is to download RedSn0w 0.9.11b1 for Mac OS X. After you make sure all the downloaded files are available, your A5 iOS device will go in DFU mode. Then just run RedSn0w. A menu will pop up. Choose Extras. Then click on the Even More option. Finally hit the Restore menu button. You will have to select your iOS 5.0.1. firmware files and match it to the iOS 5.1.1. firmware files by clicking the IPSW button.
All thatís left now is the Blobs section. For users who have saved their SHSH blobs in the past with the help Cydia, nothing more remains to be done... just click the Remote button. If you have not saved your blobs, then click on the Local button and find them on your computer. After you have done so, only 10 seconds stand between you and your downgraded device. This is a beautiful and simple release by the Dev-Team.
Source: Redsn0w update lets iPad 2, new iPad, iPhone 4S downgrade to older firmware | TUAW - The Unofficial Apple Weblog