My iPad won't charge with USB charger?

Most USB ports on a laptop or PC don't have the kind of power required to charge an iPad effciently. It will charge, but you have to put it to sleep, and then you cannot see that it is, in fact, charging, albeit at about a fourth its normal charging speed. If you try to use it while it is charging via the USB port, you will see the "Not Charging" message, because while you are using it, it is NOT charging. Expect it to take around 12-15 hours to bring it back from 20% to 100% if charging this way.

From Apple:

http://support.apple.com/kb/HT4060