Since this tread is already bumped I'm gonna give my 2 cents. Do not want to repeat my fail in the intro-forums

Anyway, as some people have said, I think special circumstances are needed for a life-debt to be created. For example, Harry dived into the chamber of secrets with the sole purpose of saving Ginny, knowing that there was a giant Basilisk down there (at least after seeing the skin) but he still did it. That is a life debt to me.

The Pettigrew thing is a little wierd to me, but I take everything JK writes with a grain of salt and don't immediately assume it is correct just because she wrote so. I mean, Harry did not do anything special for Pettigrew, he just wanted him alive so he could prove Sirius was innocent, then he was going to hand Pettigrew to the dementors to receive the kiss.

But still, this is magic we are talking about. It changes from situation to situation, just look at Harry's death and resurrection in DH.