Is there any way to restore an OLD iPhone backup?

iTunes has a nasty habit of deleting the previous backup every time it does a backup. If you really have an old backup intact, there is a way to restore from it. I forget the procedure, but I'm sure you can Google it. It's a good idea to manually go into your backup directory and manually copy it over to a new name once in a while. That way iTunes won't touch it, and you can use it in an emergency such as yours.

Here's a website that describes how to do it:

http://www.monkeedev.co.uk/blog/2010/03/11/managing-multiple-iphone-backups-through-itunes/

Assuming you have older backups:
Mac OS: ~/Library/Application Support/MobileSync/Backup
Windows XP: %AppData%\Apple Computer\MobileSync\Backup
Windows Vista: %AppData%\Roaming\Apple Computer\MobileSync\Backup
Windows 7: %AppData%\Roaming\Apple Computer\MobileSync\Backup
Go onto iTunes, right click on your device on the left-hand side bar, and their should be an option 'Restore From Backup', or something similar, click that and it'll restore your iPhone to a previous backup.