Finally, a topic I can contribute some useful info on! I have a 10D, had it since about October of last year.
I've taken it snowboarding (packed in my camelbak) up at mammoth three times so far, I've taken it packing out at Yosemite this winter and up in the Whites for a long weekend as well. I even take it mountain biking in the local mountains (Orange County).
I haven't had a problem with it yet, the chassis has been coated with a pretty good sheet of ice out at Yosemite, and powdered with the fluffy stuff at mammoth. It's taken at least one good fall when I first got it. I always strap it to the outside of my pack so I have easy access to it in case of a photo emergency. It used to swing around quite a bit when I dropped my pack, and one time it took a good little hit on the ground as I did that. Scared the crap out of me, but it never seemed to hurt it.
I think with just some basic precautions like keeping it in a plastic baggie when changing temps (into the tent after a long day) and keeping your batteries warm these cameras will do fine in most environments. It was taking pictures just fine in temperatures in the teens all winter long for me so far.
I'm not gonna start the whole Canon Vs. Nikon debate, as there is no winning that one for either side. Just remember you are buying a camera system, rather than just a body, look at the lens selection and features of the systems. Determine if Nikon's better flash is something you like, or if Canon's IS system is appealing to you, or any number of other factors on either side. In all likelihood you know this already though, so good luck. I've heard good stuff about that D70, when is it supposed to be available to the public?
If you haven't seen these sites: dpreview.com, photo.net, fredmiranda.com, luminous-landscape.com they are all really, really good places for general photo info (there are a few backpacker/climbers on the boards there too).
Here's one of the first good previews of the d70 I've seen:
http://www.letsgodigital.org/html/first ... _EN01.html
Side note, Canon is supposed to release a 10D Mark II (possibly 8m, improved feature set) later in the year. Though you can always get into a game waiting for the 'next big thing', so ignore this entire section.
Edit: a couple of my favorite pics
<A HREF="http://www.jodag.net/Mammothmoon.jpg">Mammoth Moon</A>
<A HREF="http://www.jodag.net/SA/sierramorning.jpg">Sierra Afternoon</A>