Usually, that's the bassist's job. Especially in jazz, but often in rock music, too.

Hmm.. I'll agree that with jazz music, it's appropriate to let the bass player hold the line, and let the drummer loose a little bit. But the rule of thumb I've noticed is that the more "drum driven" the music is, the more you want the drummer to keep the band in time. In rock music, there needs to be a really tight interplay between the drummer and bassist, and they sort of form a foundation on top of which the guitars, keyboards, vocals, etc. can do their thing. If the drummer and bassist drift away from each other, the bassist should follow the drummer, unless he's in some ridiculous fill, and has clearly lost the beat. In this case, after finishing the song, he should be forced to play warm-up exercises with a metronome for several days straight until he's been reprogrammed.

Of course, all members of the band have to be able to *keep* time... We're really debating which of the instruments has final veto power if things drift. And most rock is much more drum-driven than bass-driven.