It takes very little time at all for the 24 hour news networks to get started talking about the next presidential election with seriousness. I doubt that Fox waited more than a year after Bush's re-election to begin discussions highlighted with the logo "Decision 2008." Certainly this caters to political junkies that crave the discussion and speculation (if not usually die hard facts) that one hears on news programs.
It was probably before the 2006 congressional elections that the various news networks anointed the nominees. Rudy Giuliani and Hillary Clinton would battle for the presidency. Commentators probed strengths and weaknesses of both, and talked about the supposed eventual losers such as John Edwards and John McCain. Would they be offered vice presidential spots? Obama never entered any serious consideration. By 2007 McCain had dropped from the radar was well.
Then we got reminded that voters decide elections. A plain spoken Arizona senator and an utterly inexperienced Illinois senator shocked the media world.
This election may have been partly a result of the early speculation on Giuliani and Clinton. Candidacies have a momentum all their own. Some build and keep building, others build, climax, and fall. The earlier one's name comes out as a certain candidate, the more likely that voters will get fatigued on the person. Also an early entry gives ample time for the press to dig up the kind of goodies that torpedoed the Giuliani attempt.
One lesson from this campaign season is to try to avoid early discussion of your candidacy. Enter later, make sure you have no foolish blemishes to hide, and try to be experienced and public enough so that people know who you are from the beginning. That is the formula for winning this season, as we will see in November.