Check out the remarkable rendering work of Alessandro Prodan.

Mies van der Rohe Farnsworth House:
http://marlas.cgsociety.org/gallery/

CGTalk Discussion:
http://forums.cgsociety.org/showthread.php?f=132&t=754315

The plants were created with Paint Effects, converted to poly and rendered in Mental Ray. Instancing was used to keep the memory manageable. As well by keeping the history on the Paint Effects to poly the scene size did not get very large.

One problem he had with Paint Effects was getting the leaves to all face upward. The leaves normally twist relative to the way the branches and such twist. One can use the leave twirl attribute, but this simply spins all the leaves instead of orienting them to face up. The solution I found was to use the "Leaf Forward Twist" attribute. This makes the leaves face towards the camera (for doing billboard style texturing). With Paint Effects to poly the camera direction is defined on the stroke node by a physical connection to the camera location(this is "Camera Point" in the stroke mesh output block). One can break the connection to the Camera Point attribute and set it to something like 0 1000 0. It then thinks the camera is way above the trees and the leaves face upward. Obviously this is a non-intuitive workaround, so I might add an attribute to allow leaves to face the sun direction in future. Phototropism is an important characteristic of plants.

Perhaps the most remarkable thing about this scene is the lighting. The global distribution of the light looks spot on, with just the right levels of color saturation. Often with computer graphics the details can look good but the overall lighting balance is wrong so that a thumbnail of the image will sometimes look more fake than a full screen version. This scene reads very well in a small image.

My hat's off to Alessandro.