It's hard to get a substantial amount of juice from a case-mounted solar panel. Sewing a photovoltaic panel into your jean jacket will only get you so far. And while portable devices do generally use very little power, no portable solar panel has yet been able to charge a laptop computer.
But that's about to change. Eclipse Solar Gear has created a hard laptop case that can actually charge a laptop! That is, if you've got full sun and a relatively small laptop computer.