DSLRs work quite well in cold weather, but there are a few tips to keeping things working nicely from my experience.
- The most important thing about cold weather and electronics is to prevent rapid temperature changes. In most cases, this means "let the camera get cold, and leave it cold". The enemy you are avoiding is condensation, which occurs when the cold camera warms up again (particularly if it warms up quickly). Being cold won't hurt it, but cold/warm/cold/warm will cause condensation, possibly in the lens or housing which will negatively effect your pictures.
- Buttons are hard to use with gloves - setup your shooting modes before hand so you don't have to manipulate the camera much while using it. Flexible modes like aperture priority are my favorite.
- The LCD screen may start to perform poorly in cold weather - don't worry, it will be fine later, it just may not work well at the time. Again, know your controls well and setup your shooting modes before hand so you don't need the LCD.
- Your breath is a source of constant annoyance - it will condense on the viewfinder and lens and obscure your next few shots. Watch your breathing, and hold your breath briefly while shooting if you need to.
- When you are reentering a warm area, put your camera back in it's bag or case, close it all up, and let it warm up slowly over a few hours. If it gets warm all at once you are more likely to experience condensation problems.
As for the rest of your gear, most of it will be fine also, but you may want to keep spare batteries in an inside pocket so they stay a bit warmer (modern Li-On batteries actually handle the cold pretty well, but still operate at reduced capacity). No need for extremes, I just use one of the little "stash pouches" sewn into so many mid layer things these days.
Keep your lens quantity to a minimum. I know a lot of photographer's tendencies (including mine) are to bring every darn lens, but you'll take better pictures and travel lighter if you choose 1-3 lenses. I usually force myself to limit to a wide-angle zoom, a 40-50mm prime, and maybe a longer zoom, if I think there will be anything to shoot with it. If you are shooting Canon, look into their new 40mm/2.8f pancake lens - it is positively tiny, and takes great pictures (cheap, too). You'll appreciate the low weight, and not feeling pressured to constantly change lenses to get the "right" one will give you more opportunities to take pictures, which will give you better pictures in the end.
You might use one of the light neoprene cases that wraps around the camera to prevent bashing injuries, but don't worry too much about protection. If you have to dig it out of a big case every time, you'll miss good shots. DSLRS are made for professionals, and are generally very sturdy beasts. Use a good quality strap that holds the camera close to your body so it doesn't flop around, and then keep it out and ready. If it gets a few dings on it, well just post a picture of your "well used" camera and enjoy the street cred.
To integrate storage with your existing pack, look into camera bag "inserts", like the ones offered by Naneubags.com (no specific endorsement, though they are popular in the expedition world). Look for an insert that will fit some existing pocket on your bag, instead of trying to get a whole additional bag.