One thing that is crucial in black and white photography is making sure your highlights don’t get completely blow out in your quest for great contrast. Luckily the EOS 40D and 50D cameras include a Highlight Tone Priority setting that “extends” the dynamic range so that gradations between highlight tones becomes smoother. It also helps recover blown-out highlights but in my experience, the effect is subtle. Since enabling this mode limits your ISO setting from 2oo – 16oo (it also changes how the ISO looks in the LCD from 200 to 2oo so that you can easily tell it’s set), you may find this feature creates slightly more noise than you would see at ISO 100.
In my own shooting I’ve found this setting can really help out in high contrast landscape work where the highlights like these clouds below, tend to get blown out. This isn’t a fix for every situation but it will help if you’ve forgotten your neutral density grad filter. The Canon Professional Network site even includes a video “Masterclass” on this feature and it’s a great presentation to watch.