I have not done it myself but I image you would want to highlight and copy to a new layer the object(s) you wanted to be in color. Then select the orginal layer, that you want to be b&w, and desaturate it.
Well that is the way I would do it. I just did a quick test and it worked for me. The worst part is selecting just what you want and not the other bits of the background area.
Another way, and my favorite by the way in photoshop. Is to take a history snapshot of the color image. Then desaturate it. Select the history brush with your snapshot as the source and simply start painting the color back in.