Adds a custom shortlink structure to WordPress

custom-shortlink.php

<?php
/*
Plugin Name: WP Custom Shortlinks
Plugin URI: http://pmg.co/
Description: Customizes the default WordPress Shortlinks
Version: n/a
Author: Christopher Davis
Author URI: http://pmg.co/people/chris
*/

register_activation_hook( __FILE__, 'wpse26869_activation' );
function wpse26869_activation()
{
	wpse26869_add_rewrites();
	flush_rewrite_rules();
}

add_action( 'init', 'wpse26869_add_rewrites' );
function wpse26869_add_rewrites()
{
	add_rewrite_rule( '^s/(\d+)$', 'index.php?short=$matches[1]', 'top' );
}

add_filter( 'query_vars', 'wpse26869_query_vars', 10, 1 );
function wpse26869_query_vars( $vars )
{
	$vars[] = 'short';
	return $vars;
}

add_action( 'template_redirect', 'wpse26869_shortlink_redirect' );
function wpse26869_shortlink_redirect()
{
	// bail if this isn't a short link
	if( ! get_query_var( 'short' ) ) return;
	global $wp_query;
	
	$id = absint( get_query_var( 'short' ) );
	if( ! $id )
	{
		$wp_query->is_404 = true;
		return;
	}
	
	$link = get_permalink( $id );
	if( ! $link )
	{
		$wp_query->is_404 = true;
		return;
	}
	
	wp_redirect( esc_url( $link ), 301 );
	exit();
}

add_filter( 'get_shortlink', 'wpse26869_get_shortlink', 10, 3 );
function wpse26869_get_shortlink( $link, $id, $context )
{
	if( 'query' == $context && is_single() )
	{
		$id = get_queried_object_id();
	}
	return home_url( 's/' . $id );
}