WordPress MU 2.9.1 is almost ready but we need people to test it before the final release. This will be the final release before we start merging into WordPress so I’d love to get as many bugs as possible ironed out. Take a quick look at the tickets in Trac and see if you can fix any!
Only try this on a test server of course! The new version creates a new “commentmeta” table on each blog after you upgrade. That could be intensive on large sites. Ron points towards John’s script that adds those tables. I haven’t tried it yet (it’s a job for tomorrow!) but it’s definitely a good idea to create this table on all your blogs before you upgrade. Let me know how it goes.