[GEli]

I wouldn't worry too much about it. I've shot with mine in -30°C for extended periods with no trouble, aside from terrible battery life. As long as you're extra careful about banging your camera around (plastic gets brittle when very cold), you should be fine. Carry extra batteries in your pockets, and continually swap out the warm one for the cold one. Really, though, at 0°C you won't notice that much of a difference in battery life.

When you come inside - the plastic bag is an excellent idea. I use one with silica gel packets inside as well to suck up any extra moisture that might escape into the bag. Be sure to yank your memory cards before sealing the camera up, unless you feel like waiting hours to be able to review your photos. Supposedly with DA lenses the weather sealing is good enough to make the bagging an unnecessary step, but I'd rather not be the one to prove that untrue.