I've seen the hamster ball thing numerous times. it's a large sphere, hollow, on top of a set of wheels which turn and (apparantly) correspond to the game movement. It was on the science channel multiple times.
The hamster ball works exactly like a mouse if u ever look at an old mouse with the ball you will see it works the exact same way.
You could easily make a homemade version using it as a mouse input the only problem is the ball really.
I did start working on a bike riding game which i had a sensor which recorded how many clicks per second the exercise bike wheel did a full 360 and inputted that through the serial so say 5clicks a second = 3km\h or so and used that as a basis for it.
I never finished it which sucks it was ment to be for exercise but making it more fun.
I did think about having a video camera mounted on a bike and doing a long bike ride and then playing the video and the unit would either slow or speed up the video depending on the clicks per second instead of trying to create a 3d enviroment.
The hardware is possible for a lot of things problem is software most companies won't bother with building any of these things cause there isn't a market and the guys that can build it usually can't program for it
My dad was looking into buying something just like that with the speed up/slow down thing as well. It was about $3000 though, too much! I'd much rather go mountain biking at my local SORBA location anyway.