The article partially answers my question regarding why we haven't had a visit from the President-elect. It's not that he doesn't love us or love Jim Martin, but he doesn't want to risk a very high-profile (and reasonably predictable) defeat before his Inauguration. If Jim Martin loses, it's his personal loss. If Martin loses after an Obama visit, we could ask what happened to the Obama electoral magic.
But there's another possible explanation as well, one that's just a little Barackiavellian. If the Democrats have a filibuster-proof majority in the Senate, then the President has less leverage to move them away from their preferred position. If he "needs" a Republican vote or two (Sens. Snowe and Collins come to mind almost immediately), then he can press for the kind of compromise he wants. If the Senate leadership has less power, he has more. Whether he realizes it or not, he doesn't really want Jim Martin to win in Georgia.