neuroanatomist said:
Yes, it'll be a firmware cripple, same as the current optical version (the optical control signals are sent by the main flash tube).
On the other hand, maybe producing a transmitter that is able to communicate with 5 flash groups in parallel is more expensive or needs more calibration than a slave radio unit that just sits there and talks to one single master.
As I pointed out, there's no technical reason the 430EX II can't be a master, heck onboard flashes and the 90EX are optical masters. But it wasn't given that capability, to upsell buyers to the 580. The same reasoning applies to an RT version.
As for the technical aspects: No doubt you can cram in a rt slave in a 430ex body, but the question is if the performance/distance is the same as with a large 600rt unit. Seeing Yongnuo struggle with their rt clones, there might be more difficulties there than we know, esp. if you try to combine it with an optical fallback (as Canon also didn't do on their st-e3).