Dilberts - This quote was taken from the mentalimage website:
"Supported geometric primitives include polygons, trimmed free-form surfaces, hair (in mental ray 3.1), and subdivision surfaces."
Kiveun - Mental Ray does support Subdis. However it has problems with them just like it does with nurbs. The answer is the tesselation. You have to increase the tesselation factor. What you are seeing happening is Mental Ray trying to optimize the surface through tesselating. If it does not have enough value to tesselate properly then you will get what you are seeing. So adjust the tesselation value and you should be set.
I dont know about Turtle, but I do know that RMfM does do all surfaces. And not only that, it specializes in nurbs and subdis, because it ignores the tesselation values and renders it as smooth as possible. And you know how it takes forever for subdis to render, not anymore, with RMfM it is lightning quick.
Just a little bit of info.