I've seen some DIY ones using an Altoid box. They look kinda neat and simple.

Radson,

Did you use the powermonkey or powermonley with the solar battery?

Fosanna,

Do you mean\ you're taking a couple of MP3 players w/ you on the trip with identical playlists?

FortMental,

FreeCharge looks interesting as long as it can charge the ipod. It looks like you have to crank up hard to get to desired voltage on the output. What kind of mp3 players did you charg with it?

Freeplay stopped making FreeCharge 12v devices. Searched the Google w/o success

Rinat, I've seen some of those things for sale here in Belgium, so keep on looking, I'm guessing you'll find some.

Got the Griffin 4xAA battery charger (productized version of Altoids AA DIY charger) - will test it over shortly. Also as an option the idea is to take the 2GB iPod shuffle (no display, very simple) that should have pretty good standby time.