If your iPad isn't charging properly via USB cable, try changing the port you've got it plugged in to. Turns out iPad requires more power to charge than iPhone or iPod touch, and not all USB ports are powered equally (some are branched off, like hubs). This is especially true of laptops, including MacBooks. Could be front vs. back, left vs. right side, just keep trying until you find a high-power USB port that does the job.
(This is also why the iPad included a different, 10 watt power adapter from the one included with the iPhone and iPod touch). Apple has a knowledge base article up with a variety of other iPad charging related information, so be sure to check that out.
Hey, 10hrs of battery life on a 9.7" display don't charge cheap!
[Thanks to everyone who sent this in!]