Q:

Do the usb ports on the back of the XBox 360 always supply power?

I'm curious if the USB ports on the back of the Xbox 360 constantly supply power, even if the Xbox itself is turned off (but still plugged in obviously).
Also, is the power supplied by the USB ports in the back 500 mA?

A:

As long as the Xbox360 is connected to its power supply the USB ports will have voltage on them. I was able to verify this with a play-and-charge-kit that showed me the LED colour for "recharging", when my Xbox360 was turned off.
I cannot verify if the USB port will supply enough power in this state to actually charge a battery.

The power that is supplied by the USB ports in the back will surely be the same as for the front ones. USB specifications state that any high power port has to be able to deliver 5 unit loads of power, which is 500mA. Seeing as the back USB port is intended for a Wireless LAN USB connector, which usually does not have its own power supply, it has to be a high power port.
Wikipedia USB Power
USB technical documentation

