this error is present in the latest version of the plugin
it appears that around line 197 of role-scoper.php the following code is present which appears to be now deprecated in WP (http://core.trac.wordpress.org/ticket/11443)
the offending code is:
} else {
if ( ! $wpdb->has_cap( 'subqueries' ) ) {
rs_notice('Sorry, this version of Role Scoper requires a database server that supports subqueries (such as MySQL 4.1+). Please upgrade your server or deactivate Role Scoper.');
$bail = 1;
}
commenting it out successfully prevents this warning