I am having the same issues, I recently did the upgrade to snow leopard clean install, If I try and replace footage, or drag the same footage into the project window, i get that wonderful colour ball, I thought the issues was corruption because at the same time i replaced my Drives, but I did multiple tests, from new source material and the same thing is happening.
I would avoid snow leopard for now unless someone knows of a fix this means, Time to travel back to normal Os.X, Time machine would have been a good choice.