If you install iOS 5, you can't go back... maybe (Updated)
TUAW - The Unofficial Apple Weblog
Update: Some of our Twitter buddies have noted that you should be able to use the TinyUmbrella tool to kick out of the 'error 1' loop that results after restoring an iOS 5 iPhone back to 4.3.3, and proceed safely from there. Needless to say, your mileage…
7 June 2011