It's pretty clear to me that Obama talks and acts like a believing Christian, and McCain doesn't. That doesn't mean McCain is a bad person; it just means that he doesn't at all appear to be particularly religous. I suspect that this simple fact is part of the reason Obama is making inroads among people of faith. While he doesn't share the views of lots of Christians on some important issues, his overall outlook is Christian, while McCain's is much more secular. In this respect, Obama is like Jimmy Carter.
I would also add that a person certainly can be a flat out liar and a Christian at the same time--Christians are not immune from sin. I think those who accuse other Christians of not being "truly Christian" need to examine their own actions to see if they are behaving in a way that Christ would approve.