You have to choose "Set up as a new device" when restoring your iPod; if you restore from a backup it includes files (such as crash logs for jailbroken apps) that could theoretically point to a past jailbreak if Apple decided to really dig.
If you do choose "set up as new" when restoring, though, they'll never know. Once upon a time it was possible to screw your device up in a way that was both unrecoverable and easily detectable by Apple, but as far as I know that hasn't been a problem since firmware 1.x.