Here's a little contribution while my memory's still fresh. I went to Harbin in late December 2010 and the temperatures were between -25 to -30 degrees celcius. I had a D700, Sigma 24-70 F2.8 HSM, SB900 and Tamron 70-200 F2.8 with me. Everything worked fine, except for the Sigma (which was damaged back in Singapore, so that's unrelated).
The frozen D700 was working perfectly well. I wasn't.
So was the SB900.
Some tips to take note:
1) Your camera will last longer than you in the cold. Keep yourself as warm as possible if you want to take photos. Your fingers are especially susceptible to the cold and they will hurt.
2) As mentioned often, batteries don't last long in the cold. Have spares. I only experienced my battery completely dying on me once, but you'll see the power draining quite quickly. On a side note, the Phottix EN-EL3e performed as well as my originals.
3) In terms of operation, there wasn't anything unusual. There was a 7D, 450D and D60 in my tour group. The 7D didn't have any problems, but the 450D and D60's batteries died quite often. There was also one incident when the D60 had a memory card error, but the error cleared once the camera was warmed up.
4) Hold your breath when taking photos. While it may not be a necessity here, it is there, because your breath will fog up your viewfinder and you won't be able to see anything.
5) Condensation is a problem, and ziploc bags are quite a hassle. I was the only one using a ziploc bag and it did keep my camera dry while I was in a warmer environment. However, it doesn't make much sense when you're in a tour group because you don't have the time to wait for your camera to change to the ambient temperature. There was an incident where I boarded the bus after being out in the cold for two hours - condensation occured as expected - and I didn't have the time to let the condensation clear because five minutes later it was time to visit another attraction. I thought that ice was going to form on the filter, but instead the cold outside evaporated all of the moisture!
6) Using your gloved fingers to brush away snow will only melt them and make your camera wet. Use a blower (NOT your breath) to puff away as much snow as possible before boarding the bus etc.
7) Don't attempt to use your lenspen on a frosted lens. I did that and my lenspen didn't work properly after that. It made the lens (filter) worse too.
8) When it was snowing, I had my front element pointed downwards and my hand was covering the back of the camera body. That pretty much kept out most of the snow.
9) It might be a good idea to keep your camera in your bag and then take it out only when you want to use it, but that option wasn't available to me because I was using a backpack.
10) Don't be too worried about everything. The main priority is to enjoy the trip!