Hi,
when upgrading from 2.8.0 to later versions, http://yourdomain.com/wp-admin wouldn't let me log-in. Instead of a login-window I read:
"Fatal error: Class 'WP_Styles' not found in yourfilepath/wp-includes/general-template.php on line 1806"
The blog-frontend works fine. Downgrading to 2.8.1-RC1 the problem disappears. 2.8.1 to 2.8.4 seem to have the same problem.
A few blogs seem to have a similar problem, however, I couldn't find a solution yet... :/
http://www.google.com/search?q=%22Fatal%20error%3A%20Class%20'WP_Styles'%20not%20found%20in%22
Any idea? Similar experiences anyone?