I got the public_html/x/wp-admin/css/theme-editor.dev.css message
when I tried to update http://www.save-your-marriage.org automatically. However, the manual update worked fine for me.
What I did was basically what I remembered doing when I upgraded to 3.0.
I deleted the old files first, keeping
wp-config.php as it contains the configuration data
wp-content as it contains the themes, plugins, etc.
images as it contains pictures I uploaded
Then I selected everything but those three items from the new wordpress directory and recursively copied those into the root directory of the website. Also, if the .maintenance file was there, I removed that, too so that the website wouldn't keep coming up with the maintenance message.
I then ran the upgrade.php, clicked the button to update the database, and everything came up working fine.
Anyway, this site was set up to be a plain WP site and not WPMU.
I just ran an automatic update on http://save-marriages.com which I set up as a WPMU site on a VPS, and the upgrade went almost perfectly.
I say almost because it came back saying the upgrade was successful. However, when I pressed the dashboard button, I got a white screen! Yikes. I held down shift and pressed the refresh button on my browser and the dashboard came back.
Just to make sure things were consistent, I cleared my caches on the sites after doing the upgrade.