How to restore iOS 5.1/5.1.1 backup to iOS 5.0.1
For those downgrading from iOS 5.1 or iOS 5.1.1, I have successfully restored from my iOS 5.1.1 backup and kept all my settings after the downgrade.
I don't know if this is common knowledge, but just in case, here's how I did it:
(These instructions are for OS X)
1. Find your iOS backup file. (On a Mac, it's located at /Users/Library/Application Support/Mobile Sync/Backup
2. In the big jumble of files in there, find info.plist
3. Open info.plist with TextEdit
4. Locate <key>Product Version</key> <string>5.1.1</string>
5. Change "5.1.1" or "5.1" depending on which version you're downgrading from to 5.0.1.
6. Save the file.
7. Restore your iPhone/iPad with Redsn0w, ifaith...
8. Restore to that modified backup with iTunes.