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Likely, your computer does not have the required high-power USB port.
For more details, see:
and this included reference to 'high power' (defined as more than 500 milliamps output) USB
Confirming what the other poster says, my older MacBook, lacking high-power USB, does charge the iPad 2 when connected despite the message, although at a very very slow rate. I've noticed that the iPad charge increases a by percent or two when connected for an hour or more.