The solution, which I believe is only in Illustrator CS3, is to select the object or groups on the artboard you want to convert, then under the “Edit” menu choose “Edit Colors” then “Convert to Grayscale”. Simple, yes, but hidden in the menus in a very unintuitive spot.
One thing you should be aware of, however, is that doing this removes all the color information from that object/group. This means that any CMYK values will no longer be present. You can, however, always undo the action immediately after converting.