There are currently at least four Android tablets with the feature USB audio enabled:
. B&N Nook Colour with a modded Android kernel
http://forum.xda-developers.com/showthread.php?t=1459892
. WitsTech A81G with a slightly modded Android kernel
http://android.modaco.com/topic/343858-external-usb-devices/page__st__20
. Ainol Novo 7 Advanced Allwinner with a modded Android kernel
http://forum.xda-developers.com/showthread.php?p=22280857#post22280857
http://www.slatedroid.com/topic/29289-usb-audio-cards-support-drivers-and-libs/
. Archos G9 with an official Android kernel
http://www.archos.com/products/gen9/archos_101g9/index.html?country=us&lang=en
http://www.jbmm.fr/2012/01/utiliser-le-dac-fiio-e17-avec-son-archos/
http://update.archos.com/9/gen9/changes_firmware_archos_android_gen9.htm
There is an official request to Google, and therefore to Android smartphone/tablet manufacturers, to enable USB audio: like a USB mass storage device, a USB keyboard or a USB mouse, a USB DAC should be a standard USB device able to connect to an Android smartphone/tablet.
The official USB audio request is currently ranked 26 of 17969.
http://code.google.com/p/android/issues/detail?id=24614&sort=-stars&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars
In fact, it is ranked in top 3 of less than one year old requests.
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