I searched Google Images for stuff that does exist, and came up with that thing.

If this article is true, one wouldn't need usb 3.0 for gigabit ethernet anyway...

http://www.codinghorror.com/blog/2005/07/gigabit-ethernet-and-back-of-the-envelope-calculations.html

There are lots of usb 2.0 to gigabit ethernet adapters available...