Enter the "Album Artist" tag in iTunes 7. Set this tag (via Get Info with all all tracks of the album selected) to the main artist's name, and the album will be unified.
For those of you who tag with other applications, it appears iTunes uses the ID3v2 TPE2 (Band - orchestra - accompaniment) frame for this. I'll leave the debate about whether or not that's appropriate to others who care more about 100% accurate tags than I do. I'm just happy that it's finally easy to keep these albums together without resorting to calling them compilations!
Note, this information comes from a few posts in this thread on the macosxhints forums.

