Current Sportsbook Odds:
Obama -450
McCain +275
I wish Plaw were here to explain it to me yet again, but basically: Obama is the favorite. To win $100 on the bet for Obama, you'd risk $450. A $100 bet on McCain (the underdog) would get you $275 profit if he were to win.
Please don't ask me to explain that.

But you can read about it
here and maybe understand it better than I do.
You can monitor Sportsbook's odds
here, under Hot Events | Exotics: Presidential Odds