In reply to mchardski:
> Transparent plastic bag doesn't really cut it.
I put my iPhone in a ziplock bag and used it on the Cape Wrath Trail, which involved rain almost every single day. It worked fine for the entire trail.
> Have seen guides using smartphone in whiteout on glacier it in Alps on multiday tour. He had one of those portable chargers which these days are "fairly" light. He charged both that and the actual phone each night.
On the Tour of Monte Rosa earlier this month I powered an iPhone, Bluetooth keyboard and compact camera for two weeks via a solar panel and portable battery, which together weigh about 600g but lighter options exist. A battery alone with enough capacity to charge an iPhone 4-5 times can be had that weighs no more than 250g. There's no excuse to run out of power in the hills these days – particularly if you switch devices off or put them into airplane mode when they aren't being used.
For some devices, of course, you can just carry spare batteries.