Drupal Junkie here, with a Wordpress question (I know, blasphemous!) I've got a Wordpress (3.1.2) install on one of my sites, and recently had my client's IP blocked by the server firewall. Their solution was to turn off the mod_security, which completely killed the site (loading any page, including admin area, threw a 500 Internal Server Error error.
After some troubleshooting and downtime I traced down to a line "SecFilterEngine Off" in the .htaccess file. After removing that line, everything loads as normal. I hope this helps someone that is suffering from the "500 Internal Server Error" error as well!