Hey James,
I have the same problem. I updated a Worpdress site and then I got this same error.
Warning: require(/home/content/11/11352011/html/wp-includes/class-wp-roles.php) [function.require]: failed to open stream: No such file or directory in /home/content/11/11352011/html/wp-settings.php on line 125
Fatal error: require() [function.require]: Failed opening required '/home/content/11/11352011/html/wp-includes/class-wp-roles.php' (include_path='.:/usr/local/php5_3/lib/php') in /home/content/11/11352011/html/wp-settings.php on line 125
I renamed the plugins folder to plugins.hold from FTP and nothing changed no access to backend either.
Then I renamed the active theme folder to theme.hold and nothing works either.
I don’t know what else to do :S.