In theory doable, except solar cells the size of the average phone doesn't produce a heck of a lot of juice, plus the phone would need to be stowed, for a good while, where there is plenty of sunlight, which also leave is as a target for thieves.

Not if you make just the battery solar powered. Youd would take out the battery and let that be in the sun. Without the phone thievesw wouldnt take it.

Then we'd be back to 5 or 6 years ago with two or three snap on battry packs used in a day.

Good idea guys, but here again this exists already in several forms. They are all bulky becuase solar energy density is low compared to teh recharging times we all like (hours rather than days)

http://www.voltaicsystems.com/http://www.backup-power.ca/home.html

I saw this one in a magazine recently. It was a stack of solar panels that fanned out from an axis at one end. In the picture, it appeared to be relatively small. It would be great for when you're camping, etc. <tg>