Just a thought for those who will be be upgrading their WPMU installs - especially when 2.9 version is stable. This will be a major upgrade since commentmeta table will be created for each blog in database so I would expect those who have more than 12 blogs in their WPMU installs (many do) would encounter white screens at some point or another.
I experienced white screen in frontend and backend when I upgraded development install way back to WPMU 2.9 beta 2 – trunk-r2026 and ran http://mysitedomain.com/wp-admin/upgrade.php script. Fortunately, I upgraded via FTP upload that time and noticed wpmu-upgrade-site.php in wp-admin folder as I was uploading WPMU files and folders.
Running http://mysitedomain.com/wp-admin/wpmu-upgrade-site.php gave me back my frontend and backend. Check in database showed new commentmeta table for all blogs. I am using WPMU latest trunk version now and no hiccups so far :-)
