If your shooting in raw, from what i've read your image is by DEFAULT black and white, it stores all the pixels, and color information is stored separately, and added back in during post processing.
There's an episode of Pixel Perfect, Pixel Perfect- Camera Raw
where Bert Monroy talks to camera expert Jeff Schewe.
Lightroom is an excellent program by the way. I don't use it enough because I don't have a lot of memory. Whats cool, is it saves every modification you make and you can always revert back. If you take a look at the Pixel Perfect
Webpage there are also some episodes about lightroom (episodes 63/64 maybe?)