An Android 4.1 Jelly Bean device can have:
. a standard USB audio out capability (or USB audio host capability), or/and
. an Android-specific USB audio out capability (or USB audio accessory capability).
An Android device that has the standard USB audio out capability can interwork with standard/legacy USB DAC.
A standard/legacy USB DAC can be a self-powered device, or a bus-powered device.
Some bus-powered USB DAC like the Objective DAC (ODAC) can be powered by some Android smartphone like the Samsung Galaxy S III.
If a bus-powered USB DAC requires too much power, it should connect to an Android device via a self-powered USB hub.
An Android device that has the Android-specific USB audio out capability should interwork with a USB DAC of a new type.
Such a USB DAC must provide power to an attached Android device.
The standard USB audio out mode is specified in Android specifications:
"int USB_CLASS_AUDIO: USB class for audio devices."
The Android 4.0 smartphone Samsung Galaxy S III can interwork with standard/legacy USB DAC like ODAC or FiiO E7.
The Android-specific USB audio out mode is just added in Android specifications:
"Android Open Accessory Protocol 2.0 includes optional support for audio output from an Android device to an accessory. This version of the protocol supports a standard USB audio class interface that is capable of 2 channel 16-bit PCM audio with a bit rate of 44100 Khz. AOA 2.0 is currently limited to this output mode, but additional audio modes may be added in the future.
To enable the audio support, the accessory must send a new USB control request: SET_AUDIO_MODE"
Hopefully, Google and Android device manufacturers will enable the standard USB audio out mode along with the Android-specific USB audio out on their stock Android 4.1 devices. It should be available to everyone, and not only for a few ones hacking their Android device.