Recently, Barack Obama told an audience about Al Gore, "I can promise you that as president I will have him involved in our administration in a very senior capacity in his role... having won the Nobel peace prize and an Oscar that being Vice President again would be probably a step down for him."
For Gore, being someone else's vice president again might be a "step down", but being the president wouldn't. If Al Gore decided to run for the 2008 Presidential election, he would win, hands down. Clinton, Obama, and the other candidates wouldn't have a chance. People love Al Gore.
Despite the obvious advantage he has with the general public, Al Gore is (as of right now) not in the running. However, as far as I know, he hasn't definitely said no. It wouldn't surprise me if he announced one day soon that he'll run for president; it also wouldn't surprise me if he won.
Personally, I would love to see Gore run, even though I'm an strong supporter of Kucinich. It would be great to have a leader who understands the problems faced by the environment and knows what to do about it. Global warming really is the most important question posed to the candidates; after all, if things continue the way they are right now (which most scientists agree they will), within a decade or two we'll start to see the beginnings of the chaos that climate change is going to bring, like starvation, refugees, violence over resources, and more.