Recently released iOS 5.1 only have tethered jailbreak with Redsn0w 0.9.10b6, now if you want to downgrade iOS 5.1 to any previous iOS version like iOS 5.0.1, 5.0 for untethered jailbreak then guide below will help to let you do that on your iPhone, iPod Touch and iPad.
Note: You must have saved SHSH Blobs for the iOS version to which you want to downgrade from iOS 5.1.
Downgrade iOS 5.1 to iOS 5.0.1, 5.0
Open iFaith to create custom firmware and click on “Build *signed* IPSW w/Blobs” button
“Browse for saved SHSH Blobs” file for your required iOS version
Provide stock iOS firmware file to iFaith for which you have to create custom firmware
iFaith will create custom firmware and let it finish the process
Click “Proceed” button, and then click “Start” button to put your connected iPhone, iPod Touch or iPad into DFU mode by following the on-screen instructions
When your iDevice will get entered into DFU mode, close iFaith
Launch iTunes, “Shift” + click “Restore” (Windows) button, “Alt”/”Option” + click “Restore” (Mac) button and select custom iOS firmware file created through iFaith
On completion of restore process, your iDevice will be downgraded to your selected iOS version from iOS 5.1.