The crew at First Read explains that GA state law requires 50% plus one vote to win the senate race. Should no candidate reach that threshold (there is a third-party candidate in the race), a run-off election between the top two is scheduled for December 2.
I continue to be skeptical of a Chambliss defeat. Just imagine a run-off if it is literally to elect the 60th Democrat to the Senate. Will Dems turnout in the numbers necessary without Obama on the ballot? You can believe the Republicans will come out to vote.