I’m having the same issue. I’ve used PNGs for years now with no problems, and recently I’ve noticed that when saving as a png from Photoshop (cs5) and importing into Final Cut (7.0.3), the areas that should be completely transparent are now seen as solid white. If I open them in After effects, or even looking at them in quick view mode on my mac, you can clearly see the transparency as it should be, but Final Cut seems to be having trouble seeing it.
So I am bumping this with hopes that someone has figured out a solution.