Lake Tahoe late Dec 06 - Snowing and cold.
Grand Canyon later on the same trip. -20C
The H1 struggled more than the back did. Sometimes the lenses didn't want to move (for focus).
The 5D Handled well also. Not minding the cold. Taking the camera in and out of cold / hot areas produced more drama than the actual cold. I kept the cameras in the trunk (boot) to keep them at a constant temperature and stopping any fogging.