Here’s a sweet tutorial which will help those of you downgrading from iOS 5.1 to iOS 5.0.1 while restoring to 5.1′s backup.
I stumbled on to this loophole while downgrading my iPhone 4, follow the steps below, and it should work for you.
NOTE:- This requires an old backup.
Restore your phone.
Select an old backup, and select “Restore from Backup”
iTunes will prompt you to restore to the newest backup available. Allow it to do so. iTunes, in some way bypasses itself, and voila! It restores to your backup from 5.1.
Note that I was able to do this while using iTunes 10.6 and while downgrading to 5.0.1, from 5.1. This should work for all other devices, too.
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