OK a couple of answers to some of the, um, more substantive questions.
The rear panel controls do the following, according to Ayre:
Listen/Measure -- Selects digital filter algorithms. Listen "produces greater accuracy in the time domain." Measure "produces greater accuracy in the frequency domain" and provides "additional high-frequency energy".
Power Mode -- A turns the DAC on when the computer is turned on. When the computer is off or asleep, the DAC enters standby mode. B keeps the DAC in standby mode even when the computer is turned on and only wakes it when it receives data from a music player.
Disp On/Off -- Front panel display control.
Rsrv A/B -- Reserved for future updates.
As for USB-only aspect, I get the impression this was chosen for two main reasons. As an attempt to demonstrate that USB, properly implemented, can be the equal of any other digital audio interface. And to meet a price point well below most of Ayre's other gear. As Hansen put it over on AA:
Lil' Knight -- Ayre only sell through their dealer network. It's on their Web site here.