The Ecosystem Generator makes it easy to auto generate FBX items easily on using an existing mesh from within Unity.
The Modo Vimeo user group came out, so I recorded a video of making a mesh in Modo, importing that to Unity, and then throwing some foliage on to that.
Here's some renders of the example scene in the PRO version of Unity.
The unity pro version supports camera effects.
The Ecosysem Generator works in the free version of Unity without the cool camera effects.
We plan on adding many more types of foliage.
An undocumented feature is that you can paint foliage on a mask. I'm trying to work that into an editor script so that it works without having to "play" the scene.
The Unity extension is available in the Unity asset store.
I've added support to import your own foliage items.
The Ecosystem Generator also works on Android: