to get the plugin working a little bit well under WP 3.1, you (users) have to change lines near the las TODO (lines between 1124).
Replace the if statement next to ‘if( is_admin() ):’ with :
if( preg_match('|post-new.php|i', $_SERVER['REQUEST_URI']) ||
preg_match('|post.php|i', $_SERVER['REQUEST_URI']) ||
preg_match('|page.php|i', $_SERVER['REQUEST_URI']) ||
preg_match('|edit.php|i', $_SERVER['REQUEST_URI']) ){
add_action('admin_menu', 'page2cat_init');
}