From Noam Scheiber at The New Republic, "According to a source close to Al Gore, multiple people in the former VP's orbit are in touch with him on a regular basis to urge an Obama endorsement."
Gore's hesitation, according to Scheiber's source, is that he both enjoys the new contrast between himself (statesman) and Bill Clinton (hatchet man), and that he doesn't want to 'jinx' Obama.
Whatever you think of the reasons, it seems pretty clear who Gore is going to vote for in Tennessee on Tuesday. I would also add that he might be afraid that he'll endorse Obama and that Clinton would still win Tennessee, compounding the embarrassment of losing his home state in 2000.
I still believe that a post-Tuesday endorsement at this point would be more valuable than one that only has about 24 hours to sink in. I suspect Gore will endorse later this week unless Obama is blown out on Tuesday. My guess is the endorsement will come Wednesday or Thursday if Clinton is the narrow 'winner' on Super Duper Tuesday, as a check to the momentum she would have. If Obama is the 'winner', my guess is he'll endorse around Friday to carry Obama's media momentum through the weekend and the Sunday talk shows, and into the monday morning shows.