Personally, if I was forced to shoot with digital, I'd set my camera to record RAW+jpeg. That way the preview screen images (at least on my Canon) would show as black and white shots helping me get a handle on how the B&W would look. It can actually take a while to be able to visualise how something will look in black and white from just looking at a colour image.
When it comes to processing the final shot, work with the raw file, which should still have all the colour information present, and use whichever software you prefer to do the conversion - as stated ^^^ up there, a simple desaturation isn't generally enough, and you may well need to do quite a bit of work to the file to get a good result. My cheat of preference would be to use Silver Efex Pro, either straight from Lightroom, or if I needed to do a little more work, from CS5. Sadly, that little bunch of software costs more than most entry-level camera kits
Though in all honesty, I'd stick a roll of HP5+ in the EOS-3 and just shoot on film :shrug: