I remember a smart weather prop from the freestuff.  It was a simple plane, but had the property of always facing the camera.  I was never sure how this was done.  I did find though, that I could reposition and scale it so that it could be used as a background object.  It seems like maybe if the object could be reparented, but still always face the camera, it would be a great way to make useful billboards.  I'd love to see this work, as then they could be clustered to a tree trunk object...  The possibilities would be endless.