Gamertell has posted a pretty negative review of the Mophie Juice Pack that is compatible with the second generation iPod Touch.
From the review:
"Considering the battery-sucking potential of the iPod Touch with its bright screen, built-in speaker and energy consumed which moving and touching it, an external battery pack is certainly useful. Especially if you plan to play a lot of games on the go.
The Mophie Juice Pack - a fancy name for a rechargeable battery device - tries to offer not only doubled battery life but also a convenient mini-USB connection port. The battery portion works wonderfully. Unfortunately, it's the second part of the device that gave me issues."