I feel as though I have to point out that your 20D is not weathered sealed any better than the 5D, and as you say, it has performed well for you. I have both the 20D and the 5D, and the 5D has performed without problems for the (almost) 2 years I have owned it.
I have to admit to being seduced by the lure of a 1DSMkII, but after reading Randal's comments, I may just go for another 5D. At $US2600, it has to be considered something of a bargain now.
For your purposes, you might consider adding a BG-E4 battery pack... more power for cold conditions, a vertical release, and better balance to boot.
BTW, are you actually located in the Antarctic at the moment???