I have heard from various sources, including the Mission Space web site, that the Gs pulled are 3 - about the same felt by the astronauts during a Shuttle launch.
Yes, that seems low for a real space launch, but, you have to remember that the launch is a steady acceleration to orbital velocity. Gs measure the amount of force felt during the acceleration. As your body accelerates, it is travelling at a much higher rate of speed. But the rocket continues to accelerate. Since the rocket has much more mass than a person, it takes much longer for the rocket to reach higher speeds. It must continue to accelerate. Thus, the constant feeling of about 3 Gs until you are near orbital velocity.
Now that we get the physics behind the real thing, we come to why and how it works here on the ground for simulators (such as Mission Space). A centrifuge spins at a set velocity, mimicking the constant acceleration felt on the way to orbit (see above). The idea is that, if done well, the person in the centrifuge will not notice the spinning motion, they will only notice the acceleration (and with all the other stuff going on in Mission Space, it makes that feeling easier). Additionally, as an FYI, even though there is "weightlessness" (actually microgravity) in space, when there is any acceleration, inertia must still be overcome, so, when firing a rocket in space, a person will still feel a nudge, as if they were accelerating in their car from a stop.
I'll be happy to take a stab at any other physics related questions anyone may have.