As an addendum to this post, there’s a few extra tables in WPMU 1.3, which should officially be released any day now.
There are all related to the internal tag system now available in WordPress 2.3. You may also have noted that MU is now following a similar version number. No official word on how this can be used for sitewide tags, but I know some smart people are in the middle of figuring this out.
I am also working on a database diagram for the next post in this series. My skills are a bit rusty, but the good thing about database theory is that not much changes in twenty years. 😀 Just the programs involved.