QuickerTek has announced the Little Black Box II universal external battery (http://www.quickertek.com/products/lbb_II.php) . It costs approximately US$100 and purportedly adds 10 more hours of battery life to the iPad.
The Little Black Box II also works with the iPhone, iPod touch, all 30-pin iPodds and most USBS devices. LEDs indicate the charge status, and an on/off switch conserves power. Even the batteries themselves are user-replaceable.
The small size of the he Little Black Box II makes it easily transportable to provide as much as an entire day of additional battery power, says Rick Estes of QuickerTek. Like most QuickerTek products, the new Little Black Box II external battery is backed with a one-year warranty on parts and labor.