According to the Center for Automotive Embedded Systems Security, there are serious security flaws in the existing technology. Not necessarily a big deal, for now, as they observe that the risks are low at the current time. Emphasis on "current". They also state that no crackers have been observed to use the required level of sophistication. Again, emphasis needs to be on "observed". Yes, it may well be a while before automotive networks reach the point where this is exploited in the wild (at least to any scale), but I would remind you that it took Microsoft from Windows 3.0 through to Windows XP Service Pack 2 to take security even remotely seriously. That's a long, long time. And Microsoft had nothing like the install-base of the car industry. Further, the qualifications required by most companies to be a system administrator were a good deal steeper than the requirements for a car mechanic, so systems administrators were likely far more familiar with the issues involved. Also, said systems administrators are far more accountable for security issues, since there are plenty of third-party tools that novice users can use to spot malicious software.
The first question is why this even matters. It doesn't affect anyone today. No, but it's guaranteed to affect at least some current Slashdot readers in their lifetime and, depending on how rapidly car networks develop, may affect a significant fraction surprisingly fast. Technology doesn't move at Stone Age speeds any more. Technology advances rapidly and you can't use obsolete notions of progress to determine what will happen next year or over the next decade.
The second question is what anyone could seriously do, even if it was an issue. Not too many Slashdotters own automotive companies. In fact, I doubt if ANY Slashdotters own automotive companies. Well, the validation tools are Open Source. MISRA has a fair few links to members and software packages. In fact, even if developers just developed an understanding of MISRA's C and C++ specifications it might be quite valuable as it would allow people to understand what is being done (if anything) to improve reliability and to understand how (if at all) this impacts security. You don't get reliability for free, there will be some compromises made elsewhere.