I think this whole wizard's debt thing comes down to conscience, rather than some mystical, magical link. The fact that Dumbledore says that Peter owes Harry a life-debt only stems from the fact that Dumbledore has faith in the inherent goodness within Peter. After all, Peter only really returns to Voldemort because he has nowhere else to go. You can tell that at the start of GoF. I think the description runs something like "Pettigrew's face was etched with revulsion." Pettigrew does not want to be bad, he just doesn't have any choice in the matter because he is sans spine and won't face up to what he has done.

That said, I think that this whole debt thing is going to be settled with Pettigrew helping Harry out of some certain death scenario in the 7th. You know, kind of a deus ex machina thing. Maybe Harry gets taken prisoner and Peter helps him escape, or in the final battle, he distracts Voldemort, sacrificing himself and allowing Harry to strike a final, deadly blow. Something, anyway, that is going to redeem him in the end.