domain_mapping_upgrades.php

<?php
/*
Plugin Name: Domain Mapping Upgrades
Plugin URI: 
Description: Upgrade to Domain Mapping plugin
Version: 0.1
Author: Chris Morrell
Author URI: http://cmorrell.com
*/

function dm_filter_site_url($url, $path, $scheme, $blog_id)
{
	if ($blog_id) {
		return $url;
	}
	
	if (in_array($scheme, array('login_post', 'login', 'admin')) && get_site_option(' dm_redirect_admin', false)) {
		$orig_url = trailingslashit(get_original_url( 'siteurl' ));
		$site_url = trailingslashit(get_option( 'siteurl' ));
		
		if ($orig_url != $site_url) {
			if ( !in_array($scheme, array('http', 'https')) ) {
				if ( ( 'login_post' == $scheme || 'rpc' == $scheme ) && ( force_ssl_login() || force_ssl_admin() ) )
					$scheme = 'https';
				elseif ( ('login' == $scheme) && ( force_ssl_admin() ) )
					$scheme = 'https';
				elseif ( ('admin' == $scheme) && force_ssl_admin() )
					$scheme = 'https';
				else
					$scheme = ( is_ssl() ? 'https' : 'http' );
			}
			
			$orig_url = parse_url($orig_url);
			$url = $scheme . '://' . $orig_url['host'] . '/' . $path;
		}
	}
	
	return $url;
}

add_filter( 'site_url', 'dm_filter_site_url', 20, 4 );