A single USB port on a computer can put out 500 milliamps I believe. Most Macs will disable the port before damage is caused. That is why you will see low power warnings occasionally. Essentially, use hubs only for printers and other devices with separate power supplies. For the record, an iPad takes 1 Amp (~1,000 milliamps) to charge, an iPhone takes 500 milliamps, and the same for the iPod touch. Couple that with whatever you have plugged into the hub and USB controller on the computer, and you don't have enough juice.