I finally upgraded from Wordpress 2.8.x to 2.9.1. Luckily, I had checked Wordpress out using SVN, so all I had to do was issue the command: > svn switch http://svn.automattic.com/wordpress/branches/2.9/
... to upgrade to the latest version of 2.9.
I prefer to check out branches over tags because I am out of town so often and can't switch to different tags when I'm away. I have a cron job that runs 'svn up' regularly so my site is always up to date with security updates within a major Wordpress version, and I make the assumption that no -.-.x release will break my install.
Switching major versions is to big to do without my guidance, so I save those updates for when I'm sitting at my computer.
As a reminder, you should always backup your database and Wordpress files before doing any update. Alex King wrote a good blog entry about maintaining and upgrading Wordpress. It's worth reading.