Our hedemora are located at about 3,600' of altitude at a place that has a record low on this day of -33F and has an average low on this day of 13F. None of the birds have any sign of frostbite in these conditions. They range outside at will during the day and are in an enclosed, isolated coop at night. They're frequently wading through the snow. I could probably send you a free rooster or two to experiment with in your climate next year, but it wouldn't be until late spring 2012. I assume under some conditions you'd eventually see frostbite symptoms, but not so far.
I think the hedemora potentially hold the key to Northern chicken hobbyists who want a low-maintenance bird that can be productive during the winter. Our hedemora are in full lay right now, and the it's getting dark at 4:30 in the afternoon.
They work well as a stand-alone breed, but it would also be interesting to hybridize them and try to build a flock of, say, super cold-hardy birds that lay brown eggs like marans.