I went to a concert a few years ago and someone left their dog in the car, in the middle of summer, while they were watching the concert.

After my friends and I parked, we were walking out of the parking area toward the concert and we saw security hanging around this car. They did bust the window to get the dog out, while they waited for police. I don't know what happened after that, that was just what I saw.
If I saw a dog in a car, with the windows rolled up, I would call the police or 911 and ask them what I should do. They would probably recommend calling animal control anyway. The only time I take my dogs anywhere with me is if I'm going some place that they themselves can go inside. I don't take my dogs with me when I know I have to get groceries or whatever and they would have to stay in the car alone. They only time I have gone grocery shopping and the dogs were in the car was when my boyfriend was with me and he waited in the car with the dogs. Not just with heat stroke, I worry about the theft of my dogs as well. I just don't see the sense in carting my dogs all over town with me while I run errands and they will be stuck in the car.
Something else: When I worked security at my college, before I graduated, we got a call concerning an unattended child in a car in the college parking lot. When we went to the car, they mother of the child was already there, getting ready to leave. We informed her that anytime we see a unattended child in a vehicle, we have the authority to break the window of that vehicle to ensure the safety of that child. Afterwards, the police would be notified as well as Children Service and they would then take over.
Also, it is my understanding that there is NO "safe" temperature to leave a dog or a child for that matter in a closed up vehicle.