Hi,
I’ve got the same problem, white canvas and viewer on my apple cinema display. The only thing that worked for me yesterday was my opening my macbook screen and working from there, sliding the canvas and viewer across to the display (a painful way of working).
Will try all your tips and let you know how it goes. It’s not been a fun transition to snow leopard. also problems with my mobile dongle (vodafone) and a few bad crashes here and there.