In an effort to create greater access to GMAT® testing, we have added a mobile testing option to serve more schools and test takers. Now we will be able to better accommodate areas not immediately served by an existing testing center.

Starting in October 2006, GMAT® testing is literally hitting the road. You may take the GMAT® exam on a bus at select locations.

The bus route spans the country, spending one to two days at each destination. We want to accommodate as many test takers as we can in each area, but availability is limited. If you plan to take the GMAT® exam and see that we are coming to a city near you, don't hesitate to sign up. 
Where will the bus stop? 

The bus will be on the road for about six months, visiting more than 40 cities; you can view the most current schedule. The schedule may be subject to change, so be sure to check mba.com for more information.

We chose stops on the bus route because we want to make the GMAT® exam even more accessible. If there is no GMAT® testing center in your area or region, you may benefit from mobile testing.
Is mobile testing secure?

The bus is designed to provide you with the same experience you would have at any testing center. The testing environment on the bus matches the secure conditions provided in other testing centers that administer the GMAT® exam. If you take the GMAT® exam on the bus, you will be subject to the same security measures and procedures for check-in that you would go through at a testing center.

We are going to great lengths to ensure that your testing experience will not be compromised in any way.
How can I find out more or make an appointment to take the test?

You can register to take the GMAT® exam during a stop on the bus route using the same methods you'd use to schedule a test at a testing center. You will be able to register online, by phone, by fax, or by postal mail. 

There is more information about the GMAT® bus, including a list of destinations and seat availability. 
(Source and more info at mba.com)