MacWorld has a terrific explanation of the charging process on the iPad and why lots of people see a “Not Charging” message. In short, many USB hubs and ports on many Windows computers simply don’t put out enough power to charge the iPad when it is awake. However, as it sleeps it is slowly charging. Like a refrigerator light, though, you can’t tell as it displays the message when you wake it to check. Watch the battery indicator and you’ll likely see it increased.

Bottom line is the preferred charging method is the included 10W AC adaptor, followed by a high-powered USB port (Mac or some PC) or the iPhone charger. Last, other USB connectors to charge while it sleeps.

From MacWorld.Com