Oh, to fly as an eagle, among the clouds...
From the Royal Council of Matt-Cambell-Said-It-Couldn't-Be-Done:
CloudsFlythruB.mp4
These are sprite particle clouds. There is a tiny bit of a "popping" problem that can probably be solved with some R&D. (john's suggestion to turn off the sprite emitter fixes most of it) I think the potential for effective cloudscapes in A:M is there.