You probably want to look into the 'heartbeat' application for Linux. By using this, you essentially have 2 physical boxes. The heartbeat application runs on both of them and shares a virtual IP address (which is a real IP address, its just typically attached to the eth0 interface as something like eth0:1).
You can do that on systems using OpenBSD-derived "pf" without any extra applications, but I believe the original request was for having boxes in two different cities: "A primary server in Ashburn, VA and a backup in Long Island, NY." Hard to do that with a single IP address :-) due to the way IP routing works.
