Most mechanics will tell you that if the car is cold because of the cold weather outside it is okay to let it warm up at idle before you start driving it. Most of us do this to circulate the oil through-out the engine BEFORE you start driving it on a cold day. This way there is more lubrication and less chance of dry gears and pistons grinding together. So in the summer when it's hot and the oil is already at room/outside temperature it probably is safe to drive away without warming up your engine.
