This tutorial shows how you can mirror your web site from your main web server to a backup server that can take over if the main server fails. We use the tool rsync for this, and we make it run through a cron job that checks every x minutes if there is something to update on the mirror. Thus your backup server should usually be up to date if it has to take over.
Read the full thing at http://www.howtoforge.com/mirroring_with_rsync
I've been doing almost the same thing for a couple of years now - I wish the howto had been around when I started :)