Mask external WordPress links behind a redirect.

mask-links.php

<?php
/*
Plugin Name: Mask Outbound Links
Plugin URI: http://www.christopherguitar.net/
Description: Masks outbound links behind a redirect
Author: Christopher Davis
Author URI: http://www.christopherguitar.net/
License: GPL2
*/

register_activation_hook( __FILE__, 'wpse36168_activation' );
/**
 * Activation hook.  flushes rewrite rules and adds ours.
 */
function wpse36168_activation()
{
	flush_rewrite_rules();
	wpse36168_add_rewrite_rule();
}

add_action( 'init', 'wpse36168_add_rewrite_rule' );
/**
 * Add our rewrite rule
 */
function wpse36168_add_rewrite_rule()
{
	add_rewrite_rule(
		'^go/(.*?)$',
		'index.php?go=$matches[1]',
		'top'
	);
}

add_filter( 'query_vars', 'wpse36168_add_go_var' );
/**
 * Tell WP not to strip out or "go" query var
 */
function wpse36168_add_go_var( $vars )
{
	$vars[] = 'go';
	return $vars;
}

add_action( 'template_redirect', 'wpse36168_catch_external' );
/**
 * Catch external links from our "go" url and redirect them
 */
function wpse36168_catch_external()
{
	if( $url = get_query_var( 'go' ) )
	{
		wp_redirect( esc_url( $url ), 302 );
		exit();
	}
}

add_filter( 'the_content', 'wpse36168_replace_links', 1 );
/**
 * Replace external links with our "go" links
 */
function wpse36168_replace_links( $content )
{
	$content = preg_replace_callback( '%<a.*?href="(.*?)"[^<]+</a>%i', 'wpse36168_maybe_replace_links', $content );
	return $content;
}

function wpse36168_maybe_replace_links( $matches )
{
	if( ! preg_match( sprintf( '#^%s#i', home_url() ), $matches[1] ) )
	{
		$url = $matches[1];
		// http:// we'll add it later
		$url = str_replace( 'http://', '', $url );
		$url = sprintf( '/go/%s', $url );
		return str_replace( $matches[1], home_url( $url ), $matches[0] );
	}
	else
	{
		return $matches[0];
	}
}