Hello Andrew,
Did you ever manage to resolve this issue please?
I have exactly the same on a new MBP laptop.
Opening the same project on MBP (Lion) and the desktop (Snow Leopard), both with carefully color calibrated screens, the laptop canvas / viewer are dramatically darker on the laptop. Strangely though, the thumbs on the expanded timeline are identical on both machines.
Is there any minor control relating to canvas / viewer display that I am missing? It looks like a gamma issue, but settings are identical on bath machines and the laptop output a perfectly normal Quick Time Movie.
Thanks for any help,
Nick.