Customers are reporting that their newly purchased iPads aren't charging over USB on both Windows and OS X based computers. Providing some further clarification to Macworld, Apple asserts that USB charging is supported but that it varies according to the state of the iPad and the power output of the USB hub:

* For the fastest charging, use the iPad's included 10-Watt USB power adapter. This will fully charge the iPad in a few hours, even if you're using the iPad at the same time.

* When connected to a high-power USB port—such as the ones on recent Macs and the iPhone Power Adapter—the iPad will charge, even during use, but more slowly. (We haven't yet determined how much more slowly.) Some third-party powered USB hubs provide higher-power USB ports, but many don't; similarly, the USB ports on most Windows PCs don't provide this additional power.

* When connected to lower-power USB ports—those on older Macs, most Windows PCs, and most USB hubs (powered or unpowered)—the iPad's battery is not charged while the iPad is awake, but is charged (again, slowly) when the iPad is asleep. What's confusing here is that the message "Not charging" appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured, Apple says: once you put the iPad to sleep, the battery will indeed charge. (If you could see the screen while the iPad was asleep, it might even display the charging icon. It's the modern-day “Does the refrigerator light stay on when I close the door?” mystery.)

Further analysis from Apple's statements indicate that the iPad can only be charged off of USB connections that provide near 1 amp of current. The majority of USB ports on most older Macs and PCs only provide 500 mA and thus can't be used to charge the device!!!

In other words, Apple's newer computers all follow the newer 1A spec (which they enforce) whereas 'Windows machines' depend on who builds them. I used to wonder why some Dell laptops had no problems running my external 7200rpm HDD with just one USB plug but my desktop couldn't until I learnt about this...