vasikgreif on "[Plugin: WordPress MU Domain Mapping] Add a domain from plugin"

:-) Ok, short and exact answer...

I did something like this:

$wpdb->dmtable = $wpdb->base_prefix . 'domain_mapping';

        // if new domain is domain.com, set active status to 0
        if ($new_domain_host == 'domain.com') {
            $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->dmtable} SET active = %d WHERE blog_id = %s", 0, get_current_blog_id()));
        }
        // if record for this blog doesn't exist yet, insert it
        elseif ( null == $wpdb->get_var( $wpdb->prepare( "SELECT domain FROM {$wpdb->dmtable} WHERE blog_id = %d", get_current_blog_id()) )) {
            $wpdb->query( $wpdb->prepare( "INSERT INTO {$wpdb->dmtable} ( blog_id, domain, active ) VALUES ( %d, %s, %d )", get_current_blog_id(), $new_site_url, 1) );
        }
        // else update the record
        else {
            $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->dmtable} SET blog_id = %d, domain = %s, active = %d WHERE blog_id = %s", get_current_blog_id(), $new_site_url, 1, get_current_blog_id()));
        }

Seems to work fine for what I need...