If everyone else votes either Democrat or Republican, it won't matter whether Lieberman votes Democrat or abstains. If the vote for everyone else is 50R-49D, the Republicans will win regardless of what Lieberman does; if it's 49R-50D, the R's will lose unless Lieberman votes for them. Consequently, I wouldn't be too surprised if Lieberman votes for neither party.
The only way we'd end up with a 49-50-1 makeup is if Lamont somehow has a last minute surge in CT or hell freezes over and Sanders loses in VT.
Otherwise there will be two "independants" when the Senate conviences in 2007 (Lieberman, I-CT; Sanders I-VT), both of whom support the liberal position 90% of the time and have pledged to support Reid for majority leader.
In a 49-49-2 scenario, Lieberman casts the deciding vote to hand the Senate to Reid.