I am not able to charge an Apple iPad over USB with a Windows machine. But both an iPhone and an iPod charge properly over USB on this machine.
The iPad requires a higher power output than many USB ports can deliver. If you have this plugged into a non-powered hub, it may not work at all. If none of your ports can deliver a high enough power output, you'll have to use the wall charger that was supplied. You may find that some lower power ports will charge but very slowly.
The good folks at Apple have disregarded the USB standard, which dictates that devices should draw no more than 500mA of current when charging over USB. Even the iPhone and iPod draw slightly more than 500mA, though only about 30mA more. The iPad draws much more than 500mA, which is why it fails to charge over most USB connections. Apple, of course, has built support for higher charging current into it's chargers and computers, which is why the iPad charges just fine if you plug it into an Apple computer.