Per Apple’s support page, the iPad requires either the 10W adapter or high-power USB 2.0 for charging. If the iPad is very low on juice, they suggest using the 10W adapter. USB ports on Macs seem to be working most consistently. Less so on PCs. A non-powered hub, like a that on Mac keyboard, obviously won’t have enough juice, and I know from experience, powered hubs are not all created equal.
Another thing to note, which I’m not seeing in these reports, is whether the device is recognized by the computer. Presumably the first thing these guys have been doing is syncing their iPads to their computers, but since they’re jumping from PC to PC and port to port to test device charging, their iPads may not be getting recognized each time by the PCs. iPhones and iPods will not charge from a PC USB port, even a powered one, unless they are recognized by the operating system, even if only as a storage device. The same probably holds true for the iPad. If you’re testing ports, be sure your iPad is recognized by the PC at that port before writing it off.
Also, based on my experience with my AA battery-powered USB pack, I know an iPhone won’t charge unless it gets at least 5V, and it requires a bit more when it’s down to 20% or less. Using a mix of nickel-zinc 1.6V and nickel metal-hydride 1.2V AA batteries, I can reliably charge my iPhone with 5.2V when it’s in the green and 5.6V when it’s in the red (20% or less). 4.8V (NiMh only) can keep it running, but it isn’t enough to charge the battery. I’m sure the larger, thirstier iPad has even stricter requirements.