The iPad's charging challenge explained

Anyone taking possession of a shiny new iPad this weekend may have noticed an odd message when connecting their gadget to some computers or USB chargers: the words Not charging in the iPad’s menu bar. 
Confusing? A bit. Here’s a quick summary of your options, based on our experiences, Apple’s support site, and information provided to Macworld by Apple:

• 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.)

We tested our iPad with a number of Macs, USB hubs, and USB chargers. We found that while Apple's latest laptops and desktops provided enough power to charge the iPad during use, an older MacBook Pro and an older Mac Pro didn't. Similarly, while the iPhone Power Adapter provided enough power, several other USB chargers, as well as a number of USB hubs, did not.
Although we haven’t done enough testing to be sure, it appears that even though the iPad’s battery isn’t charged during use when connected to a low-power USB port, the iPad at least gets enough juice that it doesn’t have to draw from its battery. In other words, your battery level doesn’t appear to go down.
In short, there’s nothing wrong with your iPad; it’s just hungry for power. In this respect, the iPad is a lot like many USB hard drives and Apple’s external optical drive for the MacBook Air: it needs more juice than the typical USB port provides.