I'm pretty much with Jeff on this one, with a slight variation. As there's almost always some degree of limiting during the mastering process, I mix with a light amount of limiting in the master bus. Then, I remove the limiter and export as a clean stereo file. That's the file that I master.
Mastering is more than just getting the track to "sound good" if something's going to be part of an album or collection of songs; there needs to be level-matching, assembly, etc. I also run analysis on tracks, like a goniometer and such, to check for consistency. That's another reason why a separate mastering session is usually a good idea.