Originally Posted by jeffkrol 
You'd have 4 pixels per pixel so to speak. This is the concept like the Foveon (but that uses silicon depth/wavelength response so the sensors are "stacked" instead of a group of 3 (lum is missing , at least as a seperate pixel) or the Nikon patent:
Nikon's new full-color RGB sensor?: Digital Photography Review
The Nikon patent is also interesting. It has the advantage that all the photons are captured, as in the Foveon sensor. On the other hand I wonder how hard/expensive it would be to manufacture all these tiny dichroic mirrors. The proposed Pentax sensor does not seem to be much more difficult to manufacture than conventional sensors.