Been trying out Autotag ... not too bad, as reported by others earlier. Still a bit unsatisfying (the data look-up on FreeDB still sucks; UI is clunky), and the tags don't include those of iTunes3 (e.g. 'Composer' - essential for classical music). And certainly nothing interesting along the lines of 'location', 'utility' etc. - although that's maybe where the ID3 tag stops and user-based organisation starts - say, playlists etc.
[See also Ben's recent tracking of such issues at Magnetbox]
Anyway, this article on O'Reilly has a few more AppleScript-based options for cleaning up tags - and a little on the history of those increasingly important ID3 tags themselves.