Desaturating in post is easy and will get you a good result. The alternative to simple b/w materials is to copy your maps, desaturate those, then render in Poser. That'll let you retain your detail. Obviously you'll also want to convert all your lights to shades of grey as well (average the RGB values then set all three values to that number). IMHO, a good b/w image is more about lighting, shadows and contrast rather than just the simple lack of color. Don't forget to select an appropriate camera focal length for the style of image you're doing. The photography forums might be of some use to you on this one.