Is it ever safe to leave your kids in the car?

Here's the scenario: You've got two kids, both in car seats and one has to be carried. You need to run to the convenience store for a gallon of milk, or the cash machine, or the dry cleaner - each of these tasks taking, maybe, 3 minutes tops to get in, get what you need and get out. Each of these tasks have their own hassles when dealing with little ones - you need to tell your preschooler repeatedly that no, they can't have a candy bar; you have to juggle awkward dry cleaning on hangers with their large plastic coverings while trying to keep the plastic out of your baby's mouth and trying not to drop the baby or the dry cleaning.

You park in front and can clearly see the car from the window. Maybe the weather is crummy. Maybe it's not.Here's the question: Do you leave your kids inside the car with the doors locked while you run inside? Or do you unbuckle everyone and march them inside for what could amount to a 60 second errand? And what is your decision based on? Location, weather, age of the children, or other factors?

I'm genuinely interested in what you have to say on this. Discuss.