been done, spinnergy used to make tri-spoked carbon wheels and now produce carbon spoked (as in the traditional sense) The main isses were cost (as you said) plus they couldn't be repaired or trued if damaged (plus they looked crap).
A lot of manufacturers are making carbon rimmed wheels these days, they seem to be the best of both worlds, also Mavic and other road manufacturers make super expensive (IE, £4800 PER WHEEL) full carbon wheels with either hollow or bladed carbon spokes, pretty nuts really.