The iPad's USB charging caveat

Among all the new excitement from Saturday's iPad launch, new confusions over the device's ability to charge over USB have started to appear. Customers are reporting that their newly purchased iPads aren't charging over USB on both Windows and OS X based computers.

The iPad specifications page clearly state the ability to charge with USB (with no mention of OS requirements) but Apple have additionally provided a support document disclosing that "some USB 2.0 ports and accessories do not provide enough power to charge iPad."

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. 

The USB 2.0 specification provides 500 mA of current to high-powered buses and 100 mA for low-powered buses. But in April 2009, the specification gained a new "Battery Charging Specification" which provided additional current to high power devices and it seems that Apple has started to incorporate it in their newer Macs.

For example, a 2009 Unibody Macbook Pro provides 500 mA extra current when connected to an iPad:

Apple's support document states that the iPhone power adapter can charge the iPad, which doesn't come as a surprise now since it's rated at 1 amp. The included 10-watt power adapter for the iPad is supposedly rated at 2 amps. Macworld's additional tests seem to show that the iPad can keep its charge when connected to a low-powered port, indicating that it at least would not die on you when syncing with iTunes.