I just upgraded to WordPress 2.9, neglecting beforehand to deactivate my plugins. This is a mistake. If you’re using WP.org and install updates manually as I do, ensure you turn your plugins off. Also ensure you take the time to back up your site and your MySQL database. It’s worth the extra effort in case anything else goes wrong.
At any rate, after I performed the update, I could see my site but was unable to access wp-admin. The solution: I removed all of my plugins using my FTP client. It loaded without a problem after that. Then I added the plugins back one at a time until I found the offending item. In my case, it was WP Security Scan. I’m sure the developer of this plugin will release an update imminently, but it is currently incompatible with the 2.9. Once I removed it, all was well.