I don't know. I have never heard of anyone restoring to iOS 5 from the backup file of an iOS version as old as 4.1
But have seen countless problems from people restoring to iOS 5 from an iOS 4 backup, only to have to restore again, due to unfixable problems.
I don't know i iTunes would or would not allow the restoration of a 4.1 backup to iOS 5, but regardless it is just a bad idea plain and simple.