Thomas, are you talking about *purely* for viewing purposes i.e. in contact-sheet and preview windows? Or actual conversion of teh photos to B&W e.g. when copying files?
I would be interested in the former as an option, sometimes it's interesting to see color pix in B&W for editing purposes - something like a "View in B&W" tickbox on the contact sheet window. But only if it could be fairly instant for viewing only and not make actual conversions.