Arduino with motor shield controlling a continuous rotation servo. The wheel turns when marbles are detected by the infrared phototransistor and LED pair next to the wheel. Flickr set with demo video:
Nice!
Cool idea too.
Nice. Now all you need is a device to refill the marble hopper at the top 
You should have the ball pulling wheel attached to a pulley which has little cups that drag balls from the bottom of the ball pit.
Also, some sensors to check for ball flow and jams would be cool.
How about a speed sensor? Hehe
Nice one, /me
How about releasing the marbles in a specific rythm or pattern? That would generate nice sounds.
Tak-Taka-tak taka tak
Carsten