Upgrading to 3.0.6 caused a fatal error on our site too.
To get it to run again: Use FTP to rename the folder “plugins” (public-html/wp-content) which will deactivate all the site plugins and give back access to the WP dashboard. Rename the folder back to “plugins” and reactivate them from the plugin page in WP. (Do NOT reactivate WooCommerce Tax in this case.)
