deadsea - 12:05 am on Feb 25, 2013 (gmt 0)

I have worked on a very large website (top 500 with millions of pages) that keeps a hot spare of the website around at a different data center with a different IP address.

I would recommend configuring your spare to only respond to a request for the correct host name. You can do this by configuring a virtual host that is not the default virtual host. Have the default virtual host redirect to the host name. That way, the only way that this site is going to get accessed is when DNS is pointing to it. It won't get crawled and indexed by Googlebot.