Here's an interesting idea. I don't know how much power is sent through headjack plugs but this device from Inflight Power Recharger Cables will allow you to recharge USB devices by plugging into the audio jack. It has one cable that plugs into the audio jack and on the other side it's a standard USB outlet so with the appropriate adapter you can charge about anything. I imagine it doesn't charge very fast but if you're on an airplane asleep you might as well be charging your phone, ipod, Nokia web tablet or whatever. Price is $35 for the basic one with no adapters and then add about another $10 for each adapter. Or you could just go online and buy the adapters for abotu $5 ea. It also has 2 AAA batteries in it so I'm guessing it can double as additional battery power for your gadget if it's not plugged in.

I contacted them about the methods of charging the batteries and here's what they said.

"You can only power via audio input or battery input, the unit can use
any form of AAA battery. The max output is 250mA at 5v."

The only real problem I see is you can't charge your device while watching a movie on the plane, and any other time you'd have to look around for a radio or laptop in order to charge it up. It seems like the standard Wall outlet to USB backup may be more versatile everywhere except the plane. I guess if you normally charge things with you laptops USB port you could charge this and a USB device at the same time. Makes me wonder if someone makes a USB to audio jack adpater so I could charge this from a USB wall converter. Maybe I'm getting a bit carried away but I bet one could hack a USB to audio jack adapter pretty easy.

I might buy one anyway and review it just beause I'm curious.