USB 3.0 is not a totally different configuration than USB 2.0. It just has 4 more wires to tranfer data - but the original four are still there. I was in the same situation as xboxps2gba, but there are no such adapters on the market. Therefore I decided to manufacture it myself.
Here's what you'll need to do. First figure out the pin-outs. The pin-out for the usb 2.0 is easy to find on web. For the usb 3.0 pin-out you'll have to consult a motherboard manual for a MB that supports usb 3.0. You can find it on any major manufacturers homepage, e.g. ASUS. Then you'll need to get a 10-pin 2.54mm female header (usb 2.0) and a 20-pin 2.0mm male header (usb 3.0), and some wires to connect them. I got all that from e-bay. Finally, just solder the pieces together, and you'll have your own cusom made adapter!