I received an announcement recently that SpeedTree 7 was released. It hasn't been a plugin for max for several years and in fact I had forgotten about it. The last time I used it was in release 4. I recall IDV ending development as a plugin and devoting their efforts to integration with the game industry's real-time engines, something they were always doing as a high-priced license option if you wanted it. I thought their trees looked very good for what they were as opacity/billboard mapped leaves, branches and canopies. Perhaps not something best suited to Mental Ray and V-Ray but it looks like they have several products now including "SpeedTree Architect" using models from their "Model Tree Store" with the option to export as fbx for Max, Maya, and Rhino including support for V-Ray and Mental Ray. It seems the software includes a modeling application as well. There's also a much more pricey version for Cinema and VFX with some more robust options for growth and animation. Trial versions are available. Looking forward to trying it out.
Prices very quite a bit:
SpeedTree Cinema going for $4995
SpeedTree Studio (budget version of "Cinema") for $895
SpeedTree Architect for $495
Looks like you can buy individual tree models as needed for about $50 each.