The idea of RAW is that your image is not processed by the camera at all. In order to make an image monochrome, the camera would have to do processing. If your camera has a RAW+JPEG option and you choose this along with the monochrome color setting, your RAW will be full color and your JPEG will be the monochrome version.
If you're using Photoshop anyway, I'd skip the monochrome option in the camera (especially if you're using RAW) and just make the image monochrome in Photoshop. This makes much better looking Black & Whites IMHO. If I remember right, there's a post on the DPS blog from not too long ago about this very subject.
If you don't want to filter channels and just want it done quickly, use Image > Adjustments > Desaturate (SHIFT+CTRL+U).
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