What exactly does USB 3.0 have to do with charging? Will it not be backwards-compatible with 2.0?

Yes, but with 3.0 devices won't the outlet allow more power?

Er, yes: 50% more power is provided for unconfigured or suspended devices (150 mA up from 100 mA), and 80% more power is available for configured devices (900 mA up from 500 mA). Via http://www.everythingusb.com/superspeed-usb.html#4

I suspect that the outlet will give you more power than that anyway - the amperage limits restrict what devices should expect to get, not what hubs may provide them with.

You should be waiting for USB Battery Charging. That is the specification that will define how USB is used as a power source. (Greater amp and removing of different protocol limitations).