Apple has just filed a patent for an in-screen / device-covering solar film for it's gadgets. We've seen similar in-screen technology patented by Motorola already (not clear if this is an infringement) but Apple wants to cover the entire device, not jut the screen, with the solar cells.
You can already get a solar powered iPod...it just requires a larger-than-an-iPod charger that you have to lay out in the sun all the time.
Not perfectly convenient, for sure, but I suppose it's better than the regular coal-powered way of charging up. But if the solar cells were built into the device, that would certainly make things simpler.
Unfortunately, with current efficiencies for thin-film solar (the only kind that could be semi-transparent) it's unclear that there would be much benefit. Unless you were planning on leaving your iPod in the sun all day long, you'd still need to charge the old fashioned way.
Unless Apple starts making flat gadgets with a large surface area....and that doesn't seem to be the direction they're going.