If I remember correctly, the image as captured by the sensor and saved as RAW is actually B&W, with software converting it to color. Probably too simplistic, but I think that's the general idea. Not sure how much bearing that has on results. The cameras I've used in B&W mode have all saved the files as jpegs.
It seems that as far as B&W printing is concerned, the printer is the crux of the biscuit. While a regular inkjet can produce fairly decent results, a dedicated carbon black setup is more preferred. I've been fooling around with an R1900 with a CIS system using MS blacks, and while B&Ws are OK, they are just that, OK. If I had the room, I'd pick up a new printer for regular use and convert the 1900 to just B&W.