I recently met a Japanese RTW traveler on his Africa Twin that has been there in June this year. He said it was snowing on the mountain pass, but he still could ride safely. No doubt it must have been cold. I think he is the crazy one, as he was heading for Canada end November. I got a mail from him saying it was -10 degC when he was there.
So I gues it can be done, but I certainly would not enjoy it!
We've just came down from Alaska (September) and tempratures were just above freezing most of the time, that would be my limit, as anything below freezing on a bike is more like servival, not adventure. . . . . .
PS: plan to be in TDF Nov 2009.