A friend of mine has a WordPress site using the Theisis theme. Recently he added a bit of problematic PHP code to the footer via the Theme (not into a PHP file). The PHP code is erroring out causing the site to WSOD (White Screen of Death). Because the footer is on every page, he can't get back in to remove the PHP code to fix the site. I'm not super familiar with WordPress (use mostly Drupal myself) so I'm not 100% sure how to help him. I'd guess the PHP snippet got stored in the database and needs to be manually removed from there. But I don't know the WordPress schema all that well. Can someone point me in the right direction? Where in the database would this code be store so I can remove it?
Use FTP to rename the Thesis theme folder and then WP will use the default theme and you will be able to get back into admin.
If he edited the theme file via the theme editor in WP admin, then replace the file he edited in the Thesis folder from a fresh copy and try and reactivate the theme.
If that doesn't work and the added code was saved to the Thesis theme options, use a plugin called Clean Options to clear out the old Thesis theme options and start over: http://wordpress.org/extend/plugins/clean-options/