When iTunes stored album art in the music files, there was no need to share album art. Now that iTunes stores album art in ~/Music -> iTunes -> Album Artwork, if one user adds art to a shared library, it won't show up for the other user. You can solve that problem by sharing the artwork file. I put mine here:
/Users/Shared/Album Artwork/
I then replaced ~/Music/iTunes/Album Artwork with a symbolic link: Album Artwork -> /Users/Shared/Album Artwork. In order for the artwork to show up, the other users had to do an "Add to Library" and point to the shared library. Nothing gets "added" except the artwork. Note: Our shared iTunes music library is here:
/Users/Shared/iTunes Music/
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060917172124886