The Belkin battery pack one I use connects to the ipod through the charging port on the bottom and then has two suction cups that connect it to the Ipod itself. I've actually found, for myself anyway, that the extra size of the battery pack helps to prop up the ipod against the tray while flying. I think it would be a little cumbersome in any other situation but works well when sitting in a plane. I like the battery pack over other ones because as long as I carry extra batteries I don't have to worry about not having the Ipod.

I have an iGo PowerXtender. The old ones take 4 AAs, and I guess the new ones take 2 and don't have a cord - they just plug in from the top of the charger. It's $16, plus $7-$12 for the adapter, but you can use the PowerXtender with virtually anything if you get the right adapter.