I was (also) on Aconcagua in Jan 2007, but the coldest we had at night was about -20C or just below 0F. Granted, our highest camp was at 18300 feet and perhaps we lucked out with the temps. But I was under the impression our temperatures were not considered abnormal for the middle of summer there. We were on the mountain 12 nights, from 03 Jan to 15 Jan 2007.
Temps on the summit (daytime, obviously) were probably somewhere in the teens (F).
Anyway, I brought my Canon EOS 20D (DSLR) as well as a Canon PowerShot S1IS (not SLR). I carried both cameras to the summit. (Compared to my annoying size-13 mountaineering boots, the camera weight was minor.)
Both cameras worked just fine every time every day. No issues whatsoever.
The 20D had its own specific Li-ion rechargeable battery, but for all the things that use AAs, I brought lithium batteries instead of alkaline because of past problems I have had in the cold with alkalines.