Ajaxifying Comment - Add "submitting comment" message:

gistfile1.php

/**
 * Adding processing message at comment form
 * Use inline style so we don't need to load more file
 */
function simple_ajax_comment_form_mod( $settings ){
	printf( '<div id="submitting-comment" style="padding: 15px 20px; text-align: center; display: none;">%s</div>', __( 'Submitting comment...' ) );
}
add_action( 'comment_form', 'simple_ajax_comment_form_mod' );