I found this in the ipad2 manual:
Charge the battery: The best way to charge the iPad battery is to connect iPad to a
power outlet using the included Dock Connector to USB Cable and 10W USB power
adapter. When you connect iPad to a USB 2.0 port on a Mac with the Dock Connector
to USB Cable, iPad may charge slowly while syncing.
Important: The iPad battery may drain instead of charge if iPad is connected to a PC,
to a computer thatís turned off or is in sleep or standby mode, to a USB hub, or to the
USB port on a keyboard.
If your Mac or PC doesnít provide enough power to charge iPad, a Not Charging
message appears in the status bar. To charge iPad, disconnect it from your computer
and connect it to a power outlet using the included Dock Connector to USB Cable and
10W USB Power Adapter.
Not all computers will charge the iPad2 through USB.
Here's the link to the manual if you would like to read it for yourself.