It was bound to happen before long: -
Not easy to achieve technically and I can't vouch for its quality but I know two guys who have facebook "liked" this product and one of them does live gigs.
The trick is to efficiently take a slice of the 5V offered from the USB and efficiently convert it to a higher voltage that is more practical for audio mixing such as 12V or 15V. There have been really cool advances in this technology from companies such as Linear Technology, Analog Devices and Texas Instruments.
The next trick is finding the silicon (op-amps) that work close to the power rails without distorting or introducing much noise. Again there is a recent plethora of op-amps that fit the bill and 5 years ago they probably wouldn't have been avaialble. And like the power chip inverters the same companies are involved.