Keith Hill had seen mention of a standby application tier in the slides I posted and asked for more information.  I sent email to Bill Essary, and he replied with the following links covering server availability.

Ensuring Team Foundation Server Availability

You can control when you take the servers offline for maintenance. However, you must also consider how you want to handle unexpected failures. By applying one of three basic strategies, you can make sure that the servers are available to the clients during maintenance or failures. The strategy you select is determined by the amount of down-time the users can tolerate and the topology of the system.

Activating a Fail-Over Application-Tier Server

After you set up the primary data-tier and application-tier servers, you can add a warm standby computer for the application-tier server of Team Foundation.