My jailbroken backup
I made my mind and i upgrade to ios5,*
Everything run smoothly until I tried to restore from my old backup*
At the end of the process an error pops up saying this iphone could not *
Be restored, backup file is currpted or not computable !!*
I guess coz backup is jailbroken ? *
I've never encountered iTunes complaining that a backup was corrupt when the backup wasn't actually corrupt. This can be caused by a few things, but in the one time I've ever had it happen, I wasn't successful in getting much data from the backup. You can use something like iPhone Backup Extractor to review the contents and see if the backup is actually corrupt (so long as you did not elect to encrypt your backup with a password), but you may just be out of luck. Unfortunately a lot of people had difficulty doing upgrades to iOS 5. Sometimes it is as simple as restoring the same backup again, sometimes iTunes did manage to corrupt data -- especially on phones that were running the beta iOS 5 releases.
Have you tried shift-right-clicking on the phone in iTunes and restoring from backup again and then picking that same backup (or if available, the backup prior to the one that is flagging as corrupt)?
With regard to jailbreaking -- this isn't anything that is caused by jailbreaking, so that shouldn't be the ultimate issue.