3D printers might be just the ticket for small plastic trim parts or other items that don't see much mechanical stress. The printed parts I have seen have been somewhat flimsy, but the process is capable of fine detail. Knobs might be a possibility, if you had a metal strengthening collar around the shaft hole (like many originals did).
Of course, if the "Intellectual Property" crowd gets its way, they will try to strangle the whole concept in it's infancy, by forcing 3D printers to "clear" any design against a master database of "forbidden shapes" before allowing them to print:
http://hardware.slashdot.org/story/1...to-3d-printers
http://torrentfreak.com/3d-printer-d...-a-car-121012/