Yesterday I notice, even though having the iPad connected to my computer and the iPad shows not charging. The iPad is being charge. I was down to 70% charge and I connect the iPad to my pc to sync my pc bookmarks to the iPad. I forgot about it for a few hours and when I came back. I disconnected it from the pc and the iPad had 100% charge. Maybe not as fast as the power adaptor, but it does work.
When connected to 'underpowered' USB ports, it has to be in sleep mode to charge, is what I read in an article somewhere.