How to treat a camera when it gets cold and rainy is an interesting question. A bag of cold air when bringing the camera in is a nice idea as it gives it dry air to warm in, but the same bag of warm air when taking it out provides a reservoir of humidity to deposit as condensate. I wonder whether in the end the simplesst solution - no sealed bags, cushioned case or camera on the outside of your coat in winter, spare battery in pocket, isn't the least risky for a weather-proofed camera (EOS1, NIKON D). In any case it's certainly the solution which is tested most by consumers
Edmund