I just updated to WordPress 2.8 (from 2.7.1). I manually downloaded the new version, deleted my old wp-includes folder (standard procedure), and copied the new folders and files into place.
Broke everything on the back end. I was able to publicly view my site. I was able to sign in to my blog. But all of the admin areas were not responding (or rather I was getting errors).
I removed all of my plugins from the plugins folder and this returned the admin functionalities. Next was the matter of determining which of my plugins were acting badly.
One at a time I copied my plugins back into my plugins folder. I had one plugin that was breaking my admin areas (you may encounter others). My Google analytics plugin would break my admin areas every time.
Fortunately I was able to find this useful solution. Essentially, you go into the plugin files (in my case the file called sem-google-analytics-admin.php) and replace all instances of is_site_admin() with is_admin() (so remove _site from each one; I had two instances).
After making that change I was able to copy analytics over again and subsequently enable the plugin.