So the Senate is split 56-40, with four races not yet decided. Minnesota and Oregon appear to be very close, with Republican Sen. Norm Coleman less than 1000 votes ahead of Democrat Al Franken, and Republican Sen. Gordon Smith less than 1000 votes ahead of Democrat Jeff Merkley. Convicted Republican Sen. Ted Stevens has a bit greater lead than his challenger, Democrat Mark Begich — a bit over 3000 votes, out of over 200,000 votes cast — but it still seems too close to call.
This leaves Georgia, and apparently Georgia is one of the few states that requires a majority to win rather than a plurality. Republican Sen. Saxby Chambliss now seems to be at 49.9%, with Democrat Jim Martin at 47% (Libertarian Allen Buckley had 3%). So unless Chambliss goes over the 50% mark (not impossible, but not certain), there'll be a runoff December 2.
If the Republicans lose all three of the other races, the runoff will be tremendously important, since it will mean the difference between a filibuster-proof 60-40 Democratic majority and a filibusterable 59-41 majority. But of course even a 41+-member minority can only successfully filibuster to the extent that all the members stay on board. If, for instance, relatively liberal Republican Senators such as Olympia Snowe or Susan Collins (both from Maine) — or for that matter Norm Coleman or Gordon Smith, who are often seen as fairly liberal as Republicans go — refuse to go along with a filibuster, a 57-43 Democratic majority might prove very different from a 58-42 majority.
So we might have a very high-stakes (and likely very expensive) race in Georgia on Dec. 2. To be sure, the odds seem to favor Sen. Chambliss, especially since the electorate swelled likely as a result of the Obama candidacy, and many of those voters aren't likely to turn out on Dec. 2. (The Georgia Presidential turnout this year was about 3.8 million this year, as opposed to about 3.3 million in 2004.) Still, I can't imagine that either the Republicans or the Democrats will take this lightly, especially if two or three of the other races break against the Republicans.
UPDATE: Hans Bader points out that Republican Sen. Gordon Smith is now 1000 votes behind.