Here's one way to get this look...
If you're not familiar with using masks, check out this tutorial...
http://retouchpro.com/tutorials/?m=show&id=166
...specifically starting at Image 13 where it talks about selectively applying the effects of an Adjustment Layer.
Instead of Selective color, you can use a Channel Mixer Adjustment Layer instead. Turn on the Monochrome option and adjust the controls until you get the BW look you want.
Lower the Opacity of this layer for an overall look.
For areas of more intense color airbrush black on the mask where you want color from the layer below to show through. Start with low Opacity (~25%-35%) and low Flow (~30%-40%) settings.
Welcome to RetouchPRO. I hope this gets you closer to where you want to go.
~Danny~