Having made a solar-cell hat, I can tell you there are a lot of problems with photovoltaics on the trail:

1. Partial shading of the panel results in greatly reduced output. Basically, if you shade out a cell it presents a high resistance to the rest of the panel. To combat this, you must subdivide the array into many smaller panels - each a complete solar battery. Then if part of the array is shaded the remaining panels are still providing the full output voltage at reduced current.

2. As was pointed out earlier, unless you're hiking in the desert or above treeline, you'll probably be in the shade quite a bit.

3. If you stop for a while, you might find a sunny place for your panel. You need a panel that has enough capacity to charge your batteries in a short time (who wants to waste a day to charge batteries?). Said panel therefore needs to be high wattage and will be, from an ultralight standpoint, heavy (2+ pounds).

This goes back to what Dryer said - take one big battery to charge all your little ones. You can buy lithium polymer packs with high capacities for less than an equivalent solar panel. An 11 Ah @ 6V pack weighs under a pound and will charge all your gear with the appropriate charger. My PDA/phone, gps module, and AA/AAA battery charger all use a mini-usb jack at 5V. This simplifies things since I only need one charger. The only exception would be CR-123 batteries - and they're light enough to just bring extras.

Very few of us hike for more than a week without hitting a town or a cache. Consider this an opportunity to replace or recharge your battery pack.

I know solar sounds like the "green" thing to do, but sometimes it doesn't make sense when other technologies are more convenient and can do the job with less weight.

BTW, there are many companies that sell "roll up" solar panels. Here's one with grommets.