From New Scientist
Modern cars vulnerable to malicious hacks
The idea of hackers breaking into your personal computer is alarming enough. But what if they could seize control of your car’s control systems while you are driving? Using a laptop and custom-written software, security researchers have hacked into the control systems of a family car, disable the brakes and turn off the engine while the vehicle was moving.
David Levinson, Transportist 
The opening paragraph becomes a lot less threatening once they add that you need to have hardware plugged into the car as well. If someone is physically plugging objects into your car, your security has been radically compromised even before considering software issues.
(Also, spam comment above.)
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Yes, but with wireless communications, you may not need to be plugged in to be hacked if the system is improperly designed. Imagine speed throttles based on infrastructure posting variable speed limit signs, or iphone powered entry and controls. Even if this iteration is essentially harmless, the possibilities, in the absence of care, are significant.
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