I don't think it would be a usefull means of delivering a payload (explosive, chemical or otherwise), because I think it would have been done by now. Terrorists usually just park the car, get out, and make it explode on a timer or by remote control. In that case, it doesn't really matter how the car got there.

Also, I don't think it's a real problem. Hacking into the cars control elektronics is probably harder than just hooking up a big stepper motor with some gears to the steering wheel and peddals. I think I, and many others, could do that to an ordinary (automatic transmission) car in a few days with a few hunderd bucks.

I do, however, see another problem with automagic cars: when cars start to communicate their status, like speed, position and intentions, someone might be able to abuse the system to cause massive traffic jams, or, worse, accidents, by disrupting communication (unlikely, since the cars would probably have a fail-save backup system, like radar), or by relaying false information. One could, for instance, "create" fictitious cars, standing still in the middle of the freeway, tricking the cars into slamming the breaks.

I'm not quite sure how one could cause a collision, since a car would probably try to avoid anything on radar, as opposed to relying solely on the information received from other cars. Than again, radar can be disrupted as well, possibly tricking the car into avoiding a fictitious car, which in turn might cause a collision.