Happy AJAX on .com

gistfile1.aw

/**
* Generates a domain-mapping safe URL on WordPress.com
* Core's ajaxurl uses admin_url() which returns *.wordpress.com which doesn't work for the front-end on domain-mapped sites.
* This works around that and generates the correct URL based on context.
*/
function admin_ajax_url( $path = '' ) {
	if ( is_admin() )
		$url = admin_url( 'admin-ajax.php' );
	else
	 	$url = home_url( 'wp-admin/admin-ajax.php' );

	$url .= ltrim( $path, '/' );

	return $url;
}