Even so, I think the electronics that control engine timings and such (which is what has been called the "car computer" for a while now) will stay a separate system, which gets probed by the IVI system so that it knows to display the "check engine" alert.
For that matter, the average desktop computer probably has a dozen parts which are computers in the traditional "car computer" sense, and it would be nuts to consider running Linux on them (or having the main system take over their functions). You're not going to run MeeGo on your optical mouse, or have your optical mouse provide light-sensor data to the CPU; similarly, the IVI system is not going to handle all of the processing needs within a car.
There are two simultaneous processes going on: people are putting recognizable computers (operating system, multiple independent programs, user interaction, system image with processes and dynamic memory allocation and such) in more places; and people are replacing physical mechanisms with software implementations on special-purpose hardware (with general-purpose processor architectures). Both of these lead to there being lots more computers in the world, but it's the first and not the second that's relevant here.