I let a thread here about the same topic few days ago. See the thread:Chromatic noise effect in B/W with LR
Few people seem to be interested in it, even if I've posted on several forums, except an Adobe engineer on Pixmantec forum. As far as I've clear idea on the topic, the noise comes from the fact that, WHATEVER the tool for B&W conversion, we change differentially the colour layers (typically RGB) and AS A CONSEQUENCE, the color noise reduction in greyscale is MANDATORY. That was OK with LR B4, and not anymore with LR V1.0!!
Being a fan of B&W, I investigated (temporary) solutions with LR in waiting for an update from Adobe. The only one that could face to that problem consists in using the color mode and desaturate: classical way until here. Then you can use the WB & camera calibration as a raw greyscale mixer (range smaller than the greyscale mode). In that case, the noise is really tiny.
Here is a gallery where I applied the method on most of the pics: