Flamingo 2/accurender 4 both support RPC stuff i am surprised myself that Vray Maxwell or Brazil or Archvision themselves have not made something i have asked them many times.

Flamingo itself has it's own procedural plant library or you can build your own. and here are some extra plants for that from Accustudio @ http://www.accustudio.com/exchange/fm_plants.htm

maybe xfrog? www.xfrog.com

it is possible to import a sketchup 3d tree model to rhino, but its pretty huge and too many of them slows down your navigation through a scene...

If you are going for photo real I've found billboard trees to be the best, I typically use the collections from Realworld imagery + a treeline from Got3D in the back and for reflections. If you are doing billboard trees you just have to make sure they are parallel to the camera and your sun position gives you decent shadows.

I've also used some of the stuff you can make with Onyx but getting it to texture well inside Rhino and Vray is a bit of a chore.

Micha over at the Vray for Rhino forums has made a whole collection of really useful tools for rendering gathered into a toolbar.

And not to keep harshing on Flamingo, Antisthenes, but the procedural stuff in Flamingo are a great concept, but I never had any luck getting them to look decent.

Not a fan of RPC either actually...

I'm using v-ray for rhino and hoping to find something that remains on the abstract level. Nothing too blocky or cheesy though... I'll have to look into some of these unfamilar options. thanks for your comments.

also...could you direct me to the link of the toolbars?
thanks again.

and the Onyx trees without textures look quite nice in vray's GI, say if you do the clay render look.

on Xfrog Trees import the Obj and inverse the transparency map(for leaves) and then attach it to the transparency channel of a leaf mesh object by Control+shift clicking on a face