The first would be to dominate the news agenda - another good reason for him to do it today. Beyond that, its effect may not be as massive as some think. While it would certainly help Obama in California, where he still trails Hillary Clinton, his appeal is in large part to younger voters, who are signing up to Obama anyway. My point is that Gore coming out for Obama is a bit like Norman Tebbit coming out for Margaret Thatcher. It's predictable and wouldn't do a huge amount for his overall support.
The one thing it would do is make me less likely to support Obama. Although Ted Kennedy achieved that the other day!
The other big endorsement for the two Democratic candidates to chase is that of John Edwards. The pundits in Washington seem split over which way he is likely to jump. Both candidates are courting him and lacing their speeches with complimentary comments about him. Again, if he were endorse Obama today, it would further add to the 'big mo' which he is coasting with.