I've been posting about this potential feature since the pre-Froyo days, but especially since ICS made the lack of it seem an oversight. At one point, Fiio was considering making USB DAC compatibility with Android part of the E-17's feature set, but I seemed to be the only person on that thread who stressed its importance.
Nice of danielb to find such an effective way to popularize this request and mobilize those who want to be able to use a USB DAC. Starred, promoted and publicized on other sites.
The ability to use an external DAC would definitely help Android sales on this site alone. I personally am not about to buy a post-Wolfson Galaxy device until I can bypass Samsung's unholy partnership with Yamaha.
And I agree that USB is the beginning of DAC implementation. As much as I'd like to see a bluetooth DAC solution as well, a lot of people own USB DACs already, battery life can be an issue with bluetooth, and the initial cost of bluetooth DACs would be relatively high.
And think of the ultimate price difference between an iPhone and an Android device if proprietary DACs were unnecessary on the latter. Audiophiles would flock to Android and hard and software options for audiophiles would increase.