If that 3.5-hour playback time of the 30GB iPod or 6.5 hour playback of the 80GB model isn't going to be enough for you, here's the Sonnettech Volta Rechargeable Battery Pack, claiming "up to" (we're always wary of those two weasel words) 16 hours of video playback on a single 5.5 hour charge.
In addition to its extended video playback capabilities, a fully-charged Volta can actually charge your iPod three times if you can't get to a hot USB port or AC adapter. Even though it's yet another device to put into your bag of tricks, this one looks like it could keep you watching video until your eyes get crossed.
Beyond that, it can keep that iPod going for 80 hours of music listening. This is great for those of us who whine about the iPod not having enough battery longevity. If that's a priority, it might be worth shelling out $69.95.
Product Page [Sonnet Technologies, Inc.]