According to Gizmag, a team of researchers composed of two groups -- one from the University of California San Diego, the other from the University of Washington -- took on the challenge of seeing if a vehicle's computer system could be remotely compromised by hackers. The answer, in a nutshell, is "and how."
According to the report:

"The team managed to bring a wide range of systems under external control, from the engine to brakes to locks to the instrument panel to (the first to fall) the radio and its display. The attackers posted messages, initiated annoying sounds and even left the driver powerless to control radio volume.

"The Instrument Panel Cluster/Driver Information Center fared no better; as well as cheeky messages, the team altered the fuel gauge and speedometer readings, adjusted panel illumination and, in one experiment, a 60-second countdown clock was displayed on the dashboard. When time ran out, the engine died and the door locks engaged. Subsequent hacks took over the Engine Control Module, which led to uncontrollable engine revving, readout errors and complete disabling of the engine."

It gets worse, too. In a real-world driving simulation (the test car was driven on an airplane runway), the hackers were able to remotely engage the brakes, jolting the driver and bringing the car to an abrupt stop. In another test, they were able to cut the brakes completely.

The study was undertaken after the recent rash of safety recalls, many of them computer-related, raised questions as to the reliance on electronic systems in vehicles. As the article notes, a Mercedes-Benz S-Class requires 20 million lines of code -- comparable to the Airbus A380 (excluding the plane's in-flight entertainment system).

For the full (terrifying) report, see Gizmag's source article here.