Just a quick update on getting AVR-USB to run with the Arduino.
I've just managed to put together a "mini" expansion shield with a USB "B" socket and the 2 diodes and 3 resistors needed to create a USB connection to the Arduino hardware. (Have plans to upload schematic/diagram/photos eventually.)
After some code modifications (to fit using ATmega168 etc) I got Linux and OS X recognising the demo devices. I'm currently trying to get HID keyboard data to be recognised.
So, while it's not yet integrated with the Arduino libraries it's at least progress to have a shield working and the code running on the Arduino hardware.
You can follow AVRUSB on Arduino progress here.