I'd forgotten my firewire cable, and my 3G iPod was running down. I was facing an undefended airplane trip!
Panic set in, but I caught site of this at an airport store -- for $10. It worked perfectly. It doesn't lock onto the iPod so you can't wave your Pod around -- the battery will go flying. It worked well in my headphone case.
They claim 8 hours of life. Based on the size and heft I suspect it's a LiOn battery internally. I think they have a very good shelf life, so one of these will probably see you through a few separate crises. I'm carrying one in my luggage from now on!