I have been using v-ray for several years but just recently switched to mental ray. If you haven't used mental ray since Max 8 I highly recommend trying the Arch & Design shaders. Continuing to experiment with the A&D shader, I wanted to explore the translucency features. If used properly the results are actually quite convincing.
I wanted to create a leaf material that worked similar to how they are perceived in reality. If a leaf is front lit they seem very opaque and specular, but if it is back lit suddenly they take on totally different properties including translucency. I wanted to capture this and apply it to a tree.
With the A&D material, translucency is a factor of the transparency. You will only see the effects of translucency if transparency is higher than 0.0, and you will see the greatest effect if it's cranked up to 1.0.
So I set transparency to 1.0, and to keep it from refracting, I changed the IOR to 1.0 (vacuum). Also to keep it from being transparent I put a Two-Sided base into the color slot. This is what makes the translucency more believable. Of course I turned on Translucency and the value is controlled by an image. For the color of the translucency I chose a light green color based on what a leaf looks like when it's back lit.
The Two-Sided base that controls the transparency value allows each side of a face to take on different properties. For the front side, I didn't want any translucency so the transparency value is black/off. For the back I wanted the translucency to be strong so the value of the transparency is on and controlled by a gray-scale map of leaves. This keeps the leaves from being transparent.
Here is a max file (max 2008) that has a shader with the translucent properties. Just place the appropriate texture maps.