Whenever I am using my 2006 Macbook for a while, the screen starts flickering, from one shade of brightness to another. When I change the brightness, it stops. It also occurs right after startup or when coming out of sleep mode. It is running the second to latest version of Snow Leopard, though the problem has been occurring ever since Tiger.
From my experience this could either point to a defective inverter board or a simple short-circuit on the cable that runs from the logic board to the display through the display hinge. I´d suggest visiting your nearest Apple Reseller / Apple Store or (if you´re technically experienced) trying the repair yourself (a new Inverter Board will set you back around 50$, but the do-it-yourself-repair is really not that easy).