Hi
I’m looking into this now too, here’s what I found from forum posts:
[url]GeoClip Grass]
check out Craig’s link to geoClipGrass shader
Also from NVidia some interesting reading:
http://developer.nvidia.com/content/gpu-gems-2-chapter-1-toward-photorealism-virtual-botany-0
http://http.developer.nvidia.com/GPUGems/gpugems_ch07.html