Why you guys have to get frustrated with something that is clearly not designed to do serial enumeration? These USB atmel devices need custom code to deal with high speeds, given that they don't do any other work in the background.
There are plenty arduino boards still available with FTDI USB interface. If you want my oppinion, the arduino guys are trying to cut the cost in such level that they go to second quality solutions. I mean these atmel USB microcontrollers are great but in the robotics area that are mostly used they are not required to transfer large chunk of data so fast! (9600bps at rate of 10 updates per second in most cases!)