The pay grade line is a good excuse to use when you just don't want to deal with the question. But it only works when there's actually a higher pay grade above you. So I think Obama deserves to be mocked for using it at the Saddleback Forum. The President of the United States doesn't have a boss he can pass tough decisions to.
I would be thrilled if presidential candidates used some variant on that phrase to say that some issues are outside the jurisdiction of the presidency. "Education? If I knew how to teach kids better I'd be running for the school board. Why don't you ask me about something that the president is actually responsible for?" But I've read Obama's Blueprint for Change and it's quite clear he considers all aspects of human behavior fair game for presidential intervention. Not that McCain sees the office as more limited--he just has a shorter to-do list.