Before this whole thing slides too far afield, I want to throw in a few points to consider.
It takes about three weeks to acclimate to extreme heat. At least, according to my doctor. We were discussing exercising in the summer heat around here (south MS), how so many people have told me that I was putting myself in danger by doing so, and how I'd be fine with water and electrolytes so long as I was properly acclimated. By exercising or working outside year-round, acclimation happens automatically and safely. If you're tossed into a non-A/C world suddenly, it is a serious problem.
In a disaster, people who aren't acclimated to heat suffer doubly because there's usually heavy work to do. For the physiological reason just given, but also because many aren't used to any sort of manual labor. I saw this first-hand after Katrina. And brother, there was plenty of manual labor and heat to go around.
You have to know how to deal with heat. Hydration, how to dress, dealing with non-opening windows, etc. Some years ago I was in Ottawa in August on a business trip, during a "heat wave" naturally. I put that in quotes, because it was just normal south MS weather for my co-workers and me. Still, every evening there were several heat-related deaths reported on the local news, all of them tragic and most of them preventable. (But don't put me in Ottawa winter weather, I'd likely be dead PDQ too. I am not looking down on Canadians here!) Similarly, after Katrina I saw a lot of misery because people have lost a lot of hot weather coping know-how, and many newer houses just aren't set up to live in without A/C.
The elderly tend not to drink enough water, and are more sensitive to temperature problems. Tragic, and they always seem to be hit the hardest in black-outs and heat waves. Very young children too, but with proper adult care they seem to do a bit better.
Back on topic, I can see where even a short grid-down situation that exposes people to unaccustomed heat could have a stunning death toll, even on reasonably prepared rational folks. Being able to deal with extreme heat is in its own way a serious prep item, every bit as serious as the familiar beans-bullets-bandaids list. There is no substitute for acclimation, which comes at the price of sweaty outdoor work and exercise. After that, there's a whole body of expertise (when to work, when to hydrate) and hardware (openable windows, fans, camelbaks, etc.) that have been allowed to slide in this age of easy and inexpensive A/C.