Thus far, I believe televised debates in the presidential campaign have all been happening on cable news channels and, thus, in low definition. General election debates will, by contrast, be shown on the networks and thus in high definition in those households with HDTVs. Given the age disparity between the candidates and McCain's history of skin cancer, I think one has to believe that more pixels-per-inch isn't going to serve McCain's cause very well. We'll be living in an era when only a minority of households have HD, but it's plausible to imagine that those who do and those who don't will come away with different impressions of the event, like the radio/TV contrast in the Nixon/Kennedy matchup of 1960.
UPDATE: It seems there's a CNNHD which had broadcast high-def debates. I just haven't seen it because Comcast doesn't carry it.