I’ve run into the issue before, when you think you can use a powered USB hub as a power source for the Raspberry Pi and as a hub to connect USB devices to the board. Unfortunately, as Bharath Bhushan Lohray explains, “USB hubs are supposed to only power up the ports to the full 500mA capacity on connecting and enumerating a USB device. In the absence of a USB device, the ports are powered to output no more than 50mA—just enough to enumerate a newly plugged device.” This means that it won’t provide the power needed by the Pi.
Bharath offers a hack which can get around this problem. On his site, he details how to mod a freebie bus-powered USB hub to act as both a power supply for the Pi and a powered hub for USB devices. It’s perfect for any project where you have many USB devices (and therefore use a lot of power), but only want a single power supply for the project. Needless to say, this hack isn’t limited to the Raspberry Pi and and will work with any single-board computer.