Don't Run, Al
If Gore did enter the race, I would probably root for him (I'm registered independent, so I can't actually vote in the primaries). I'm not especially impressed with any of the current candidates. However, I think it would be better for Gore and for the country if he doesn't run.
During his political career, Gore was cautious and calculating, and often disappointed environmentalists. But after giving up politics and moving to the realm of activism, he has blossomed. Many people read this as a personal transformation, such that Gore 2008 would not be the awkward and centrist candidate of 2000. I think it's much better explained as an effect of the context -- as an activist, Gore does not feel the need to please voters or make pragmatic compromises to get policies enacted. So he's free to focus on inspiring people and making a passionate case for what he sees as the best policy. Returning to electoral politics would throw him back into that compromising mode, deflating his persona (and perhaps leading to a dissipation of his support much like fellow drafted Tennessean Fred Thompson). What's more, a presidential candidate must have positions on every issue. But an activist can and should focus on one or two issues that really inspire them -- in Gore's case, primarily the environment and secondarily the war. The shift to a more well-rounded portfolio of concerns would further deflate Gore's aura.
There is a necessary symbiosis between activists pushing for change and politicians carrying out the logistics to make that change happen. While Gore would be superior to any of the other candidates on the political side, he stands nearly alone in terms of environmental activists with his international stature and access to power*. The law of comparative advantage thus says that he would do more good in his current role, standing outside but bestowing advice and endorsements upon the political work of other candidates and officeholders.
I also think that entering the race would retroactively cheapen everything that Gore has done since losing the 2000 election. One of the key storylines about Gore up through 2000 was that he was a congenital politician, hungry for power, who had been running for president since he was born. His activism in recent years can be read (i.e. in my second paragraph) as striking out in a new direction that is more productive for himself and for the country, and which expresses a genuine commitment to addressing the problem of climate change. But it can also be read as an attempt to rebuild his brand in preparation for a third shot at the White House. If Gore gets back into politics now, that second reading will become very powerful. It would cheapen both Gore -- who would be seen as phony and grasping -- and the environmental cause -- which would be seen as a pseudo-problem whipped up to feed a demagogue's ambition.
As for whether Gore will run, I think even Gore himself isn't completely sure. In 2003, he took himself out of consideration early and decisively. This year, he has tried to downplay speculation about a run, but he has refrained from making an unequivocal statement of non-interest, either publicly or in private to the people expending much effort and resources in the Draft Gore movement. My sense is that he leans against running, but he's leaving the door open while he watches the dynamics of the race. One appealing scenario is that if Barack Obama can make up some ground in the polls and Hillary Clinton's environmental proposals prove lackluster (Obama's, from the little I've read so far, are fairly good, though he lacks the easy command of the issue that Gore has), Gore may throw his endorsement behind Obama.
*I'm trying to phrase this carefully so as to make clear that Gore's role within environmentalism is unique and important, without disparaging other environmentalists who are doing excellent work in other roles within the movement.