Finally, after what seems like an age, the download page has been updated with the new WordPress MU 1.3 release.
This is a sync of WordPress 2.3.1 which includes native tagging support as well as many bug and security fixes.
WordPress MU specific features include:
1. Better admin controls for the signup page. It can be disabled in various way.
2. Upload space functions have been fixed.
3. The signup form is now hidden from search engines which will help avoid certain types of spamming.
4. Profile page now allows you to select your primary blog.
5. Database tables are now UTF-8 from the start.
6. If you're using virtual hosts, the main blog doesn't live at /blog/ any more.
7. The WordPress importer now assigns posts to other users on a blog.
8. A taxonomy sync script is included in mu-plugins but commented out. It hasn't been tested much but if your site has many hundreds of blogs it might be worth spending some time on a test server. Replicate normal traffic patterns and see if the server can cope with the upgrade process. If not, then look at the sync script, uncomment it and iterate over all your blogs with a script.
Developers - get_blog_option() will never return the string "falsevalue" again. That bug has been squished and it now returns the boolean value false.
Extensive documentation is being built up on the WordPress MU Codex by many people, including Martin Cleaver who bugged me about moving the docs from Trac and about telling everyone that documentation help is always needed.