I think the ultimate sollution would require quite a bit more engineering, but i've got some very intresting ideas.
It'd be very easy to splice an RC/Avation 11.1V 340mAH battery pack into the dock connector.
This would just about tripple the battery life of the iPod, I imagine. And the battery pack itself is a little over 80 grams, and 9/16th (12mm)of an inch thick. And 45mm (2 inches) tall) It could fit in it's carrying case and a doubling of the battery life isn't bad. Just connect the + and - pins into the firewire power pins of a male connector and you're good to go.
If more power was needed, progressivley larger battery packs could be used.
Unfortuatley, the charger for 11.1V lithium polymer packs is 100$ or more.

But think of how much iPodding you could do with a 11.1V 700mAH battery pack.

(Hint: 24+ hours of battery life.) And these could be connected outboard to a belt connector so it wouldn't decrease the pocketability of these guys.
