The relationship between Petigrew and Harry is yet to be explored and holds heaps of interesting possibilities I reckon.
If the creation of a wizards' life debt only takes helping people out of a potentially deadly situation, wouldn't that mean that everyone in the world owes Hary a life debt?? Therefore, we have to assume that a specific protective act by one wizard to another will create a debt. This means that Petigrew owes Harry and that Snape owed/s James (saving Snape from going into the jaws of a werewolf-Siirius).
Does Voldemort owe Harry a life debt because he used Harry's blood to reserect himself??
What will that mean when Voldemort comes to try to kill him??? Can he?????