I'd look at the hanlin v3 by jinke. I also do a lot of backpacking, mountain climbing, sailing, etc, and it's what I'm thinking about buying. Most companies don't seem to care about the kinds of things that I consider important.
My motorola phone for one died just from going in and out of airconditioned rooms. they wouldn't fix it because it was "water damage" even though that water came from normal condensation.
The hanlin v3 specs actually specify acceptable operating temperatures, which most devices don't even bother advertising. They specify operating temperatures from 0-40 Celsius, and (battery removed) storage temperature from -20 - 55 Celsius. That's pretty impressive I think. It's probably usually above 0 in your tent when you're reading right? at least once you've been in there a while? I'd hope it's usually above negative-twenty-celsius in your backpack...
Another nice feature is that the (user replaceable) battery is the standard and widely available lithium ion nokia phone batteries (there are plenty of third party knock-offs too, they were ~$3 in the Philippines)... so you just buy a spare phone battery anywhere, and make sure you keep it above zero degrees somehow... and it's li-ion per your requirement.
With the eink technology, it won't hurt your eyes, and one battery will last a long long time.
I haven't bought one yet, I'm trying to decide between the hanlin and the cybook, but I haven't decided yet. One issue you might have with the jinke/hanlin is that you can't buy DRM books. you can copy txt files and html files and mp3's and images and rtf and pdf and most other formats you can think of, ... but you can't buy books with encrypted content management in them (yet.. hanlin is working on mobipocket support).
I'm sort of thinking it might be better to resist the temptation to buy proprietary formatted encrypted books anyways, ... because if, for example, you bought a sony or kindle and froze it, and then you wanted to be able to read your paid-for content again on some other device, you'd be limited to buying only the exact same device. If that device isn't produced anymore, then you lose all the books you paid for.
At least with the mobipocket DRM format (supported by cybook, and planned for support on the hanlin), you can read those books on multiple kinds of devices.
Unencrypted non-drm books include pretty much every book where the copyright has expired, you can download them from the gutenberg project or other places. If you like reading classics then they're freely available. If you want to read new york times best sellers released last month (and don't care how long you get to keep them), then having the proprietary DRM content will be more important to you.
Or you could go with the ruggedized PDA's, but those will hurt your eyes more and take up more batteries.