When the first wave of iPads launched less than a month ago, some users complained about problems with charging their device.
The issue is this: Many USB ports, even USB 2.0 ports are not high-powered enough to charge the iPad.
If you own a Mac, most of Apple's more recent computers will meet the requirements. However, some may not. Unfortunately, the only way to find out is to plug it in. If it didn't work, the iPad will let you know with a message that says, "Not Charging."
Apple has already explained the problem, and the iPad will not charge via USB if in use. The iPad must be in standby mode. Also, the computer must be awake. Sorry, no sleep for your iMac until iPad is fully charged.
Even when it is charging, it will be a slower charge than if you connect to an outlet with the included 10W power adapter (the best option). Those who own an iPhone can also choose to use the small wall connecter that comes with that USB cord, but the result will still be a slower charge. Connecting your iPad to the USB port on an Apple keyboard will do nothing aside from telling you the port is too low-powered.
A few other notes:
1) This may seem obvious, but newer users may not know it. The more charge/recharge cycles you run your battery through, the less battery life you will get each time. That explains why a 2-year-old iPod doesn't last as long when charged as a brand new iPod.
2) Remember that old thing everyone used to say about having to fully drain your battery before recharging? Well, that doesn't happen anymore. At least not with Apple's batteries. One charge cycle may mean two charges or more.
For example, say you charge your iPad to it's full capacity. Then you watch Star Trek and have about 60 percent battery after the movie. Charging your iPad then only makes for half a cycle. If a day later you use another 40 percent, then recharge, you have completed one full cycle.