Voting Rights Watch: Why such a large undervote in Georgia's Senate race?

Bloggers in Georgia have been questioning the large undervote in the Nov. 4 Senate race. As Jay Bookman reports over at the Atlanta Journal-Constitution, there are some 168,785 missing voters in the Senate race. That would mean that more than 168,000 Georgia voters went to the polls on Nov. 4 to cast ballots for president, but did not cast a vote for the highly contentious race between Saxby Chambliss and Jim Martin. With an undervote of 4.3 percent, it's the largest out of any of the other Senate races.

As Bookman asks:
So who were these people? Were they Obama voters who just cast their ballots for their favorite and walked out? The evidence for that is weak. In Fulton County, which went for Obama by more than 2-1, the undervote was 2.85 percent, lower than the undervote rate in McCain counties such as Cobb (3.4 percent) and Cherokee (3.1 percent). In DeKalb County the rate was 4.4 percent, about the state average. So what's YOUR explanation?
Even though the undervote mystery has not been solved, the Senate race is schedule for a Dec. 2 runoff, and remains one of 3 still unresolved key races that could mean almost total control of that legislative body by Democrats if they can attain 60 seats.