Most powered USB hubs will not feed power back into the host system. This is normally a good thing. However, it is possible to power the Raspberry Pi by back feeding power from a powered USB hub.
Note: Doing this bypasses the fuse on the Pi, so it is possible to damage it if too much power is drawn into it. Since all my peripherals will be connected to the hub and GPIO will not be used, then there is little risk of this occurring. Another note: The rev 1 Raspberry Pi boards have fuses on the USB ports. These will provide some protection but will also reduce the power. I expect that a typical powered USB hub will have adequate power such that this will not be a problem.
Below is a simple modification to make a USB hub backfeed. The power connector on the hub I used has +5V on the outside of the barrel, and ground on the inside. There are three connectors on the power socket - one ground and two power connectors. When the power plug is inserted then power is drawn from the power supply instead of from the host. This can be overcome by simply connecting the two power connections on the socket.
Now the 2A power supply for the USB hub will power all of the periperals as well as the Raspberry Pi itself.
I need this much power in order to connect an external hard drive. The purpose for this system will be described in my next blog post.