Cycles (the new Blender renderer) has an awesome ability to chew through Geometry.
It leverages the power of your video card to perform floating-point operations
(in parallel, no less) and performs really well (but your screen will be locked
up completely, as your GPU is busy with other things).
So it seemed to be time to really get to grips with it. I used this tutorial:
After 50 passes, there was still grain, here after 150 passes some remains.
Conclusion: materials with nodes are tricky, also Cycles is still changing.
[Best viewed at 100%, there are actually some boots back there!]