This idea has no technological flash but what I wouldn't do for an actual button on cell phones that could be easily engaged for when you are driving, preferably hit before but easily tapped while driving if you forgot beforehand. It would let all attempting callers and texters know that you are driving and unable to respond, maybe even silencing the ring to avoid driver's curiosity.
I am in and out of vehicles a lot throughout the course of a day both within and out of the city. Today's society is totally demanding about how fast they get responses, sometimes becoming frustrated with the shortest of downtimes. Phones and driving ups the distraction anti no matter how amazing of a multi tasker one thinks they are. Bluetooth can help with phonecalls and verbally translated texts but really can we not occasionally just tell the world "hey I'm driving, cut me some slack". An option I'd love to have, and who knows maybe more people than we know have respect for people's driving time if they just knew they were driving.