I think the real problem is that the translucency for a leaf is not just a single flat color. Usually when you look at a leaf with a light shining through it you see a complex pattern formed by the veins in the leaf. Back lighting really does nothing to make the leaves look realistic, since there are no options to use a function on it and there is no way to vary the intensity of the backlighting. Actually a better way is to come up with a custom clip map with patterns on the clip mask for the leaf and put this mask in almost every channel: color transparency, etc. This would make the leaves look better by default. Also another thing I noticed is that using backlighting also exaggerates the edges and increases rendertimes, especially for forests from a distance. Turning off backlighting is a lot better. The best option would be to have a translucency channel added to the material editor. Wish I had thought of it for the wish list.....