I have a MacBook Pro with both Mac OS X Lion and Windows (under BootCamp) installed. While in Windows, I tried to charge my iPad 3 via the USB port. The iPad says "Not charging", but really, it does, just super slow (4-5% per hour). The reason for this is that the iPad requires 2.1 A of current, but a standard USB port provides 0.5 A.
However, if I switch to Mac OS X, the iPad states that it is charging and it does at a rate of about 10% per hour.
The only takeaway from this is that either OS X puts out more current on the USB port or that it recognizes the iPad and acts accordingly.
Can someone provide more information on what is actually happening?