I've been trying to re-read all the Harry Potter in preparation for the release of number 7, so I'm fresh on the story. I had forgotten that at the end of Harry Potter and the Prisoner of Azkaban, Harry and Dumbledore have a talk about the fact that Harry spared Pettigrew's life, thus allowing him to escape and return to help Voldemort. Dumbledore reassures Harry that the consequences of his charity are unpredictable, but that more than likely, he'll be glad to have Voldemort's servant be in debt to Harry Potter.
This amounts to about a page worth of book, so it's too important to ignore. I'm now calling that in the final showdown between Harry and Voldemort, Pettigrew will either save Harry or kill Voldemort, or both.
Or was this already obvious to everyone?