According to Fairvote.org, John McCain supports instant runoff voting. Or at least he did back in 2002.
I think I first liked the idea of IRV because it greatly increases the chances that someone like John McCain would get elected.
He is so much more rational than the other candidates, is very respected by people of both parties and frequently disagrees publicly with his party when he feels that the issue is important.
I think we'd all be a lot happier with the outcome of elections we could choose a candidate that was more in tune with the majority of the people. Systems like IRV and approval voting bring us closer to such outcomes than the existing system.