I got a comment from one of the blog visitors on How to upgrade from WordPress 3.0 to WP 3.0.1 (manual & automatic to latest version) saying that he is getting this error when trying to do the automatic upgrade from wordpress admin panel:
Downloading update from https://wordpress.org/wordpress-3.0.1.zip…
Unpacking the update…
Verifying the unpacked files…
Installing the latest version…
Could not copy file.: /public_html/wp-admin/css/theme-editor.dev.css
Installation Failed
This error is due to the permissions issue. It’s not necessary that the folder(directory) or file is unwriteable or not having 777 permission, important thing is if the apache user has permission to modify the files.
On some server, if a file is uploaded by ftp user (using ftp client) then the apache user (http client) don’t have the permission to alter that file.
Solution is to try the manual upgrade of wordpress in this case (if you are sure that the error still persists even after setting directory permissions to 777)
Note: Deleting the upgrade folder in wp-content and then retrying auto update helped some users too, so do give it a try.