Not sure if it was the upgrade to latest WP version 3.1.3, or upgrading the plugin to simplemap Version 2.2.4.
As soon as the plugin is installed I get this error message:
Fatal error: Call to a member function get_default_options() on a non-object in /var/www/vhosts/lusanbidets.com.au/httpdocs/wp-content/plugins/simplemap/classes/options-general.php on line 52
Of course this means you cannot even access WP admin; you have to remove the plugin via FTP in order to regain access.
Rolled back to simplemap version 1.2.4 and no problems!