XPI Power has released a small, US$4.99 adapter for the iPad: the iXP1-500 (http://www.xmultiple.com/xwebsite-powercircuit-xpower1.htm), which is designed to solve the problems iPad users have charging from their non-Mac desktop and laptop computers.
You can attach the iXP1-500 adapter to your existing Apple charge/sync cable, and your iPad will charge from your computer’s USB port. Plugging an Apple tablet into a computer’s USB port, instead of an AC jack, often results in syncing only.
The iPad requires 10W of energy to charge, which is much more than a typical USB port provides. However, even a USB port will work extremely slow compared to an AC outlet. That means most desktops and laptop USB ports won’t charge the iPad. Therefore, to charge the iPad you have to be a slave to a wall outlet.