Absolutely no flickering cloud shadows for me
Barely noticable shimmer to tree shadows. Only real problem is flying through clouds when effect is a bit like looking at an old TV picture that flickers horizontally. I think that is unrelated to shadows on ground and more to do with the prop effect they put back into the game.
Edit - Just installed latest nVidia driver and tree shadow shimmer almost completely gone - have to stare hard to see it. Prop flicker in clouds better but still there.