Title, required, one.
I think that dear Tim has a point, but it's the same that all practitioners of ``computer security research'' share: It's easy to break into systems that never were designed with security in mind. That includes the most widely used desktop eye cand... er, warm body entertainment software.
Though like with various SCADA systems that are increasingly attached to the wider internet in an insecure fashion, cars are ever more networked. If people haven't managed to scribble on the dashboard via the bluetooth carkit, then that doesn't make these findings purely theoretical.
To put it in perspective, however: It never was very hard for a dedicated attacker to sabotage a car and turn it into the owner's coffin. There has always been an interest in such, and now it's out in the open that you can do it electronically, too.
This won't affect most people too much as that sort of attack by nature is something likely targeted at one specific person. Altough I'm sure there'll be former-east-block ne'er-do-wells who'll figure out how to hold some upmarket brand cars ransom over the remote maintenance features. That'd be somewhat novel.
Looking forward, I expect this to rear its ugly head again so about when car-to-car communication becomes ubiquitous. You know, systems that'll let the car in front of you warn your car to start braking NOW because in ten miliseconds it'll start braking to avoid colliding with an upcoming traffic jam. Because these systems clearly are not secured in any way, driving trough a bad neighbourhood might get an extra dimension as it could give your car viruses like it ran windows.