AWESOME idea. I love it.
My only comments: Remove the tube, it won't be needed. The pingpong ball can ride an air column just fine... the column pushes the ball back to center. Remember your middle school science teachers doing this with a vacuum cleaner? If I remember right, it's bournoulli's (?sp) law in play, air rushing around the curved surface creates a pressure well or something like that; I'm sure you can find the "why" out there on these here interwebs, but skipping theory, it works in practice so the academics aren't strictly necessary.
That, and then color the balls with fluorescent paint and front-light it with either a standard black light or some high-power UV LED's.. I'm already visualizing a spectrum analyzer made with a series of these.... what's the "response time"? No matter what, the fall rate won't exceed gravity, how fast does a 0%-100%-0% cycle take?