It seems certain third-party iOS USB cables have issue charging the 3rd generation iPad. I am actually using the factory 10 watt wall charger that came with the new iPad. I got two of these cables from this eBay seller:
When I plug the cables in to the Ipad it says "not charging". The cables do work for a iPhone 4S and iPhone 3G, however.
Another old third-party USB cable did work with the iPad, so it seems to be an issue with the cable.
Can anyone recommend another eBay seller that sells cables that work for the new iPad?