Is he up for re-election?
I think he won in 2002, so not until 2008.
Yes, his seat is up in 2008. The outgoing GOP gov of Montana in 2002, Marc Racicot, would have been favored to beat Baucas in 2002 had he entered the race, but he said he wanted to go into the private sector to make enough money to provide financial security for his family. You can't fault a man for that, but I wonder if he will consider a run in 2008? I know some have said he had RINO tendencies, but as Baucas' vote against Alito proves, a little bit of conservative is better than none at all.
There has also been speculation that Baucas will retire once this term is up, but its also thought that significant Dem gains this Nov would convince him to run again. The best case scenario is for the GOP to at least hold their margin this year, and thus possibly dishearten Baucas and convince him to hang it up. But an open seat would be no guarantee for the GOP, as Conrad Burns struggled to win reelection in 2000, (and that was before the taint of scandal was on him), and a Dem won the governorship in 2004 in spite of Bush's landslide win in the state.