@oliver: in a GPU Gems there is an article about vegetation vertex animation. maybe in part 3, which can be read on NVidia site (but no source code as I remember). basically your scene looks very good for me, only some tweaking needed already mentioned above.

Edit: the gpu gems stuff is fine for small oscillations of leaves, and looks fine when combining with the original 3dgs wind oscillation. just implemented (not all features of course).