As I see it, the likelihood of a successful dark horse candidate is next to nil. A dark horse occurs when no consensus can be reached...usually at an open convention. But there are no open conventions, only dog and pony shows for preselected candidates from the primary system. In past years, primaries settled on the nominees by mid to late summer. In 2008, it is possible--perhaps even likely--that the nominees will be selected by Spring. This transforms the election season from a weeding out process leading to the convention to an eight month one on one campaign.
This means fewer chances to hear alternative ideas and have alternative choices. This means that the challenger only needs to attack the incumbent without offering ideas of his/her own. This means that every vote an incumbent makes during this silly season is predicated more on how it plays with the constituency, less than being a principled position.
Anyone who has aspirations for a presidential try, needs to have declared...yesterday!
BTW, California, as I understand it, will hold a Presidential primary in February, a general primary in April, a run-off in June and the national election in November. No wonder nobody takes election seriously enough to show up at the polls.

John OTSC (who has only missed voting in one primary or general election since 1961)