The battery is quite obvious to pick out of this picture of the rear case (sans display assembly).

The 3.75V, 24.8 watt-hour (6.5 Amp hour [!]) battery provides the juice for an advertised 10 hours of use. In contrast, the iPhone 3GS has a 4.51 watt-hour battery and the MacBook Air has a 40 watt-hour battery.

The battery seems to integrate two separate 3.75V lithium polymer cells wired in parallel for such ample battery life.

The USB power supply had to be enhanced specifically for the iPad. It's a 10W unit manufactured by Foxlink Technology, Ltd., part #A1357 W010A051.