there is this:

http://www.gadgetreview.com/2006/03/...d-battery-pack.

I have one and it's terrific. My iPod will run for 24+ hours with this (actually, I've never had the iPod run out of juice, so I don't know how long it will work), plus it protects it, too. Even nicer is the rubberized jacket which prevents the iPod from sliding around on a tray table.