A team from the University of Southern California has discovered a way to spoof the sensors for various warning systems in modern cars remotely.
http://www.theregister.co.uk/2010/08...wireless_hack/
Now, this does create a serious worry. While all they have done is spoof warning messages from the tire pressure sensors (which are compulsory in the US and apparently may be here soon as well), if the techniques developed were placed in the wrong hands, they could theoretically be used for some very bad things.