The GeekPod 100 from BatteryGeek.net provides 100 hours of iPod goodness.
Only problem is the foreign charger - so I am hoping for a UK version at some point.
Edit - Further to Michael Harrison's comments about Batterygeek. I have started to think that a homemade solution maybe a better way to battery power the iPod.
The solutions offered by Drew Perry and Chris Diclerico (Verson2 and Version3 seems a reasonable way to approach this.
The 8 hours seems a reasonable amount of time to get the iPod back to a power source and extra batteries could always be carried.