Gravatar comment verification...

gistfile1.php

<?php

/**
 * Check whether a comment author is using a valid gravatar email address, if he/she is, approve the comment.
 *
 * @param bool   $approved  Whether or not the comment is approved alreadt.
 * @param array  $comment   The entire comment object.
 *
 * @return bool  $approved
 */
function yst_check_avatar_exists( $approved, $comment ) {

	if ( !$approved ) {
		// The input is expected to be slashed, so strip those and then do an md5 calc.
		$hash = md5( stripslashes( $comment['comment_author_email'] ) );
		// ?d=404 makes gravatar return a 404 when the gravatar doesn't exist, so non-existent gravatars will not return a 200.
		$resp = wp_remote_head( 'https://secure.gravatar.com/avatar/'. $hash . '?d=404' );

		// If the return code was a 200, the gravatar must exist, so approve the comment. 
		if ( !is_wp_error( $resp ) && $resp['response']['code'] == 200 )
			$approved = 1;
	}

	return $approved;
}
add_filter( 'pre_comment_approved', 'yst_check_avatar_exists', 10, 2 );

I like this.