We took a roll-up solar charging panel with us to Peru. And ended up neither needing it or using it.
But … if it was built into the backpack itself. We certainly would have.
TreeHugger.com posted a short video with Shayne McQuade, inventor of the Voltaic Solar Backpack. Very interesting.
Looking forward to hearing from those who have tried this technology on the trail.
There are a number of different models available. This pack costs about US$220.
besthike.com 
Can’t you attach the rollup to your backpack somehow? It’s a hard thing to have to buy a new backpack (and a specific model) to get this functionality.
The one we had in Peru was quite light and fragile. It would not have been a good idea to tie it on to the pack.
We planned to use it at camp, but ran out of light and time each day.
I purchased a little one, about the size of a VHS tape which recharges 4 AA’s. I intended to take it hiking with me, but ended up upgrading to a camera which used a proprietary battery. I used it a little while driving around NovaScotia. Not a real test. I will be taking it on my next hike because I have once again upgraded my camera, and am now using double A’s again. Looks pretty simple to strap on the top of my pack, and even a little charge is better than nothing. Will report if I ever get any useful data/results.