The USB out on the apple tv is inactive. However, there are several how to's on the net about how to hack and install the full version of osx on the apple tv. Has anyone tried this? I would imagine if you could install osx you would be able to set the audio to output via usb which you could then connect to the dac. The process to hack the unit isn't all that simple but might be worth the time if we could connect the dac via usb.
There is also a company that can increase the hard drive to 250gb. I'm about to try that as my first gen 30gb model is stuffed to capacity and holds about a quarter of my music. Streaming directly from itunes works well but the computer has to be on for this to function.
http://www.techrestore.com/xcart/product.php?productid=17964&cat=356&page=1