It's certainly not a technological barrier, since I assume the 600EX flashes are backwards compatible with all the other ETTL camera bodies Canon has ever made.
That doesn't seem to be entirely clear yet:
"Sync Speeds — I shot at all sync speeds, up to 1/8000″ in High-Speed Sync, on both my 5DM2 and my 60D. I used both a 600EX-RT Speedlite and the ST-E3-RT transmitter to trigger. In hundreds of frames over the past week, I have not had one sync issue. The manual says that on pre-2012 cameras (anything other than 1D X and 5DM3), the system takes a one-stop hit in sync speed and that HSS is not possible. This will be an interesting area to watch as others begin to use the gear."http://pixsylated.com/blog/canon-600ex-rt-first-impressions/
Looks like it works even though the manual says otherwise. And there were a few other features that are 5DIII/1Dx only, like the control over the 5 wireless groups.
It'll sure be very interesting what this all means in the end, what other products this may trigger, if there will be any other RT flashes from Canon, what PW et al will do next, etc etc.
I'd also like to see a list of countries where they won't be able to sell these - and where technically speaking you're not allowed to use the new radio system. I'm still very surprised that Canon did this even though the latest PW without any TTL capability could have been an indicator of something like this coming.
I'm much more fascinated by the flash and the remote than the new camera (as nice as it looks of course).