DOWNGRADE iOS 5.1 to 5.0.1 for UNTETHERED JAILBREAK:

For downgrading to iOS 5.0.1/5.0 custom firmware using iFaith

Step 1: Simply download the latest version of iFaith for Windows from here, official stock iOS 5.0.1 firmware from here.

Step 2: Build a signed firmware file by clicking on ‘Build *signed* IPSW w/Blobs’.

Step 3: Browse for the SHSH blobs file that you previously saved for your current device to downgrade back to the old 5.0.1/5.0 firmware. Next, iFaith will ask you to select the stock firmware of which you want it to create custom firmware file from. Once done, iFaith will have a new custom firmware file for you waiting on the desktop with SHSH blobs incorporated for the firmware you are downgrading to.

Step 4: Next, enter DFU mode using iFaith and then simply use iTunes to restore to this custom firmware file.

Step 5: -> Jailbreak iOS 5.0.1 now.

Done.

P.S. As mentioned above, if you have both APTicket+SHSH blobs saved for iOS 5.0.1 or 5.0, you can downgrade your device to a custom iOS firmware.
This however wont work on iPhone 4S and iPad 2.
Only iPhone 4, 3GS, iPad 1, and iPod touch users can downgrade to iOS 5.x custom firmware using this method.