Check if a shortcode exists in WordPress

shortcode.php

<?php
if ( ! function_exists( 'shortcode_exists' ) ) :
/**
 * Check if a shortcode is registered in WordPress.
 *
 * Examples: shortcode_exists( 'caption' ) - will return true.
 *           shortcode_exists( 'blah' )    - will return false.
 */
function shortcode_exists( $shortcode = false ) {
	global $shortcode_tags;

	if ( ! $shortcode )
		return false;

	if ( array_key_exists( $shortcode, $shortcode_tags ) )
		return true;

	return false;
}
endif;
?>

Excellent! Just what I was looking for.