If I were writing something like this, there'd be a very clear demarcation of "safety critical throttle/brakes/steering stuff" | "blinky lights and radio"
I suspect that it starts out that way when the engineers are arguing about the liability issues of going full-throttle at random, but then it becomes standard, everyone gets a bit blasé, and before you know it the assistant vice-chairhorse of marketing is talking about how cool it'd be if your radio gets louder as you speed up.
Even then, I'd be arguing in favour of something like a data diode[1]. I can't think of any non-critical system that should have input to the cruise control[2]
[1] https://secure.wikimedia.org/wikipedia/en/wiki/Unidirectiona...
[2] Of course, if it did, I'd claim it was by definition now a critical system. :)
As an ex-blinky-lights-and-radio tech, I can assure you that they are clearly separated.
Admittedly, my experience is limited to BMWs, but there's enough shared knowledge across the industry about the Right Way to do this stuff that I'd be surprised if Jaguar had made that mistake.
Cruise control malfunctions can be relatively benign or they can be the equivalent of someone stomping on the accelerator.
The real hackers are screwing around with (hopefully their own) ECUs: http://ecuhacking.activeboard.com/