"been burnt by politicians "
The real point is that Al Gore has not been a politician the last 6 years. After a short period of soul searching following the 2000 election he went back to doing what he cares most about: warning people about the dangers of global warming. He's being himself now, not a politician running for president. A lot of people like what they see.
Here's a good read:
http://www.time.com/time/nation/article/0,8599,1622009,00.html
He seems to be edging a bit closer to running since this article, but it could just be wishful thinking on my part.