Do the ultrasonic sensors get used at speed? It was my understanding these are for parking only - though perhaps they're used for blind spot monitoring on some vehicles too? I've had my car parking sensors activate in traffic for unknown reasons, usually it thinks you're moving at a low speed so parking mode on, but they seem to be a bit erratic when in traffic, nothing is too close but they beep randomly. Maybe that's the reason?
For the radar, I would guess it is some kind of repeating code. I've had a pondering for a while as to whether you could "virtually" brake check another vehicle by driving in front of it, picking up its regular radar pulse, then simulating a return pulse that indicated you were braking hard, without actually braking. This would of course have implications for safety as accidents could be triggered remotely by a malicious individual with nearly no evidence. If the radar manufacturer has any sense, the code will be generated by an encryption algorithm that's hard to predict, making this kind of attack impractical; something similar to how the remote key fobs work. If they're not so smart, then the code will be fixed for each vehicle, so at least code collisions are unlikely. If they're clinically stupid, it'll be fixed for all models of that vehicle or radar module, and the vehicle will respond only to the strongest signal. But I really can't see a product like that shipping as there would be too many incidents.
If you have a varying code, you can use something like CDMA to separate your code from what otherwise appears as noise. And the radar modules are already doing a great deal of DSP on the signal to use the return pulse. I don't think it'll be as simple as FM radar, as that would probably defeat something like CDMA (you couldn't distinguish your pulse from others) and you're more interested in the time measurement.
Incidentally, the radar modules have to filter out all stationary objects (as road signs, manhole covers, etc.) have a relative speed of zero, but this makes ACC systems vulnerable to completely stopped traffic. (i.e. coming up to stopped cars in fog, or driver not paying attention...) I don't know if any manufacturer has solved that yet.