From Mark via the wp-hackers mailing list:

Big features to test:

• Basic image editing (rotate, flip, resize, crop)
• Post/Page image thumbnails. Enable the admin UI by declaring support
  in your theme: add_theme_support(‘post-thumbnails’);
• Trash, with undo functionality, for posts, pages, comments
• Comment Meta table and functions — like Custom Fields/postmeta but
  for comments
• Easy media embeds, oEmbed — paste a URL on its own item and have it
  turn into embed code
• register_theme_directory() which enables plugins to bundle their own
  themes, without copying (BuddyPress, primary example)
• Combo upgrader — get notified of plugin updates in the WP core
  upgrader, as well as being informed of crowd-sourced compatibility
  information for the plugins.

It’s bug-fixing and polishing time! Our priorities should be, in this order:

1. Fixing regressions in old features/behaviors
2. Squashing bugs in the new features
3. Polish

Additionally, today WPMU has caught up and released 2.8.6. From the website:

This is a security release with the same fixes as WordPress 2.8.6 plus quite a few MU specific bug fixes too.

From the WordPress.org blog:

2.8.6 fixes two security problems that can be exploited by registered, logged in users who have posting privileges.  If you have untrusted authors on your blog, upgrading to 2.8.6 is recommended.

On another note, WordPress 2.9 is still scheduled for a final release at the end of November/early December. You can still download and test the latest WP 2.9 nightly build here.

Finally, WordPress MU received a few updates last week and is now up to 2.8.5.2.

If you run into this problem and you don’t have access to phpmyadmin (or don’t want to give users access to it) the Edit Category Slug plugin is your answer. Works flawlessly in WPMU 2.8.4a.