Although the timing is surprising, Gore's decision isn't.
The cynical read is that Gore feels betrayed by Clinton/Terry McAuliffe and sees them propping Wesley Clark, so Gore is putting his weight on the candidate most likely to break up Clinton's hold on the Democratic Party.
The Rasputinian read is that Donna Brazile (by the way: she actually reads email sent to her) made up her mind to support Dean several months ago and has pushed Gore to make this move.
The futurist read is that Gore has always believed in the power of the Internet to transform politics and is giddy that someone has come along to actually do so in this nation (the Net and other modern digital communication nets have had a profound effect on the politics of other nations).
The straightforward read is simply that Gore agrees with just about everything about Dean, from policies to message to campaign tactics to the supporters, and has come to the realization that if he is involved in this election cycle, he would be dishonest if he didn't admit that he only likes Dean.
The comedic read is that Dean promised Gore that he could be his vice president.
I suspect that the reality is some amalgam all of the above (except the last) and other factors, with a heavy emphasis on the straightforward read.
For example, if you go back to Gore's MoveOn speech, in every way it closely resembled a Dean speech more than any other candidate. Gore simply finds a real kinship with the man.
But it is surprising that it's this early. My take on that is that Gore wants to give Dean the most time possible to campaign against Bush rather than the other Democratic candidates.