I'd link it if I could find it, but the best analysis of why Al Gore hasn't -- and almost certainly won't -- run for president this year came from Ezra a while ago. It goes like this: As long as Gore stays out of the race, all the other Democratic candidates know that his blessing would provide a dramatic boost to their campaigns, while his curse could break them. So they'll make sure to do right by him, proposing ambitious plans to fight global warming. But if he gets into the race, there's no gain for the other candidates in being really progressive on environmental issues, because all the environmentalist support will go to Gore anyway. The strategic calculus then favors finding some way to undercut Gore's favored proposals. In the end, the best way for Gore to shape the race is to stay out of it.
In keeping with some of the things that Matt says, I don't think it's especially likely that Gore will endorse before the first primary. But if we get a post-Iowa situation where the field has been narrowed down to either Obama or Edwards against Hillary Clinton, a Gore endorsement two or three days before a major primary could decide the race. (The recent Friends of the Earth endorsement for Edwards makes me optimistic that he might be the one to receive Gore's blessing.)
