Having had my odometer roll over quite a few times since I regularly commuted in all weather, I'm a bit more selective about riding conditions (no ice or snow, broken bones ain't worth it), but there is no such thing as too cold (around here anyways). Salt is the biggest problem - it just eats steel parts such as chains, derailleur pivots, cables, etc. I guess you can put on a Wippermann stainless chain and go all titanium with the rest of the bike, but for that kind of investment you might as well just fly first class to Australia and ride in summer sunshine.