I was going to announce WordPress MU 2.9.1 today but I knew that people would run into trouble with the missing commentmeta table if they didn’t upgrade their blogs immediately.
So, download add-commentmeta.txt, rename it to add-commentmeta.php and copy it into your mu-plugins folder. Login to your site as a Site Admin, visit Site Admin->Upgrade and upgrade all the blogs on your site. Make sure you’re using WordPress MU 2.8.6 as the upgrade script in older versions may not execute the plugin.
The script above will add the commentmeta table to each blog. Give it time because it will take quite a while on large sites. WordPress MU 2.9.1 tomorrow.