using an insulated bag to keep MB cool.....

Nice theory but I would caution you to make sure that it's cooled down before putting it into the insulated bag. Realistically, the hot summer car isn't a good idea. Electronics don't do well in extreme heat. This holds true for notebooks, cell phones, camaras, etc. Then there's the theft component... It's just to risky.
My suggestion is a slim bag with a shoulder strap and tote it along on your errands. This will eliminate both the heat and the thievery elements. I use the [STM alley small laptop shoulder bag|http://www.stmbags.com/catalog/macbook-bag-13-inch/alley-small-laptop-shoul der-bag> and it works very well. -GDF

The best thing for it is to be out of direct sunlight. Storing it underneath a seat should be enough.

If your vehicle is extremely hot, you shouldn't leave your machine in the car anyway--as GF mentioned, extreme temperatures can do more harm than good to a computer.

~Lyssa

The actual storage specifications for current MacBooks and MacBook Pros is -13° to 113° F ("storage" not "operating"). The inside of a car an easily exceed 113 on a hot sunny day in Maryland.