My guess is that Gore has made the only player's move open to him, endorsing the guy who was political plutonium in the mainstream Democratic party, and political dynamite to the young left (plus the beard-and-ponytail boomers). Looking at the mainstream field, Gore was frozen out as far as being a power broker. The legacy of 2000 isn't a winning issue for New Democrats; and those to the left, like Gephardt and Kerry, didn't want to be playing Gore's avenger. But there is a group with whom the "stolen election" issue resonates, and most of them are in Howard Dean's camp now.
By endorsing Dean, Gore is betting that a Dean victory, perhaps even just a Dean nomination, will wipe away the Clinton machine, including Terry McAuliffe. If Gore had instead sat on his hands in 2004, he'd be a party nobody by '08. This move keeps his name in circulation. Besides, Gore has no interest in the party status quo, since that only benefits Hillary. His better bet is a tidal shift toward a more populist Democratic party (where Al was going in 2000, and where Dean is today). In that light, it's his best choice, since the other option is irrelevancy.
More: Sullivan has some thoughts on who benefits and what this does for a DLC anyone-but-Dean movement. Start here, then read and scroll.