I am looking forward to having a reference implementation for how an on-chip USB perihperal would be used within the Arduino environment (requiring some sort of cooperative tasking beyond what currently exists, if not an actual RTOS.) But I don't think that it will be "easy", or even close to easy...
But we kind of already have that today. Go have a look a Paul's teensyduino USB implementation.
Isn't it closed source ?