Didn't it also turn up earlier in the movie, in the scene from Snape's memory? It's not in that scene in the book, but I thought it was a nice touch to put it in there, suggesting that it was something Sirius said a lot. In the DoM scene in the movie, I can't make up my mind whether Sirius actually thought for a moment that it was James (as Molly thought he was doing earlier), or it was his way of saying Harry was like his Dad.