As for condensation, the camera is double bagged in ziplock sandwich bags. I'm not taking it out of the bags until it's back up to room temp. The LCD displays seem fine at this temp.
The ambient air in the camera and bags could still form condensation. If the cameras operating it'll probably be OK (since it's warm and the condensation should form elsewhere first) but some desiccant packs in the bag wouldn't hurt.
Hacki is right about battery performance. AFAIK, both NiMH and li-ion will stop working right around -20. The suggestion of insulating around the camera is good. With even a fairly modest amount of insulation, the heat generated by the camera should be enough to keep it from dropping that low.
One easy (but potentially messy) way to get form fitting insulation is to use expanding foam in a plastic bag.
You'll also get a lot more life out of the battery if it isn't dropping to the very limit of it's operating range.
edit:
BTW, here's another user working on high altitude flights:
A550 - Script both stills and video?
