Men With Pens has a great post about WordPress 2.8: It’s Time to Upgrade Your Site. They describe what’s new and also reminds us to deactivate plugins before an upgrade.
I was in a rush when upgrading one of my blogs and did a ‘deep end of the pool’ upgrade, upgrading WordPress without deactivating my plugins. The result was that all the site showed was an empty page. It turned out that one of my plugins (Redirection) was outdated and did not work in 2.8. Removing that plugin folder gave me back a working blog.