Just a warning for anyone updating to WordPress 3.0, there are at least 2 plugins that will break it (that I'm aware of)
Events Calendar 6.6-beta
Podcasting Plugin by TSG version 2.3.9
If you have a problem trying to get into your site. access the site via FTP and remove these plug-ins. If you still have a prblem remove all your plug-ins and add them back one by one till you find the one that fails.