In its blog, Consumer Reports warns that early iPad users may be unable to charge the device via USB in certain situations and devices. In reality, even an Apple article details this problem, but I imagine that the main factor for the consumer protection publication to manifest itself can be summed up in just one word: PCs.
Not teasing who will buy the tablet and do not have a Mac (before), but the Consumer Reports tried to charge it with four PCs via USB 2.0 and was unsuccessful. When it comes to charging the iPad through a computer, only Macs are able to do this and only when the gadget is connected directly to the machine, that is, it would also not receive power through the USB port of an Apple keyboard.
In case of doubt, it will be better to use the device's power adapter, which allows you to recharge it through a common outlet. For more details on the topic, visit this Apple support article.