"We need a thread on the smallest and lightest cell phones"
Catch there is that small/light must be traded off versus delivered functionality, and different people will weight the value of different functions differently. I wouldn't consider the iPhone, clearly another person finds it an excellent option. Neither of us is wrong, we're just differently weighting the various functions versus various other factors including weight, cost, complexity, quality/reliability, etc.
"Five days in "Airplane Mode" (WiFi, Phone off) and 154 pictures later, it had only used about 80% of battery."
I've done a few power tests with my Glofiish phone, and camera use was the least power hungry of anything I tried. From a full battery charge I took 50 outdoor pictures at max (2 megapixel) resolution, turning off the unit between every three shots. At the end my battery meter still read 100% (rounds to the nearest 5%).
Indeed, having any of wi-fi, bluetooth, GSM phone, or GPS active sucks battery power, keeping those on only when needed is an important part of power management.
"I've had very poor luck with ATT coverage in the Sierra backcountry (as a former subscriber) so that would be a deal-killer for me."
The related question is whether any other provider would give better coverage --- if so, I've not heard. A few weeks ago I did some ad hoc analysis of the coverage maps of the majors (ATT, T-Mobile, Verizon, Sprint), basically plugging in various zip codes in more remote areas along the PCT. ATT and Verizon came out best. I can't use Verizon as only ATT and T-Mobile support GSM. To be clear, this was just an exercise looking at coverage maps, but in multiple cases I would find that a particular place I plan to walk through on the PCT has no coverage shown by Sprint or T-Mobile, but some coverage by one or both of ATT and Verizon. YMMV indeed (!).