I don't have a 20D, but it sounds to me like your camera is configuring your RAW files in a way that CS2 can't read.

Just to clarify the problem, when you say "open them in CS2", do you mean Photoshop itself, or Adobe Bridge? And when you say "RAW viewer", do you mean the Canon software, or CS2's RAW viewer?

I guess what I'm wondering the most is why you'd even want to shoot in camera in a BW mode. There are many ways to convert a RAW color image to BW in CS2, and they all offer way more control of the image.