USB audio is enabled on Texas Instruments Blaze tablet, which is targeted at developers.
http://omappedia.org/wiki/USB_on_Blaze_Tablet_with_ICS
http://www.svtronics.com/products/5-blaze-tablet-1-development-platform
"The two HS USB ports are the standard High Speed Host ports and are used for connecting devices like Mouse, Keyboard, USB stick, Camera, USB audio Headset/speakers etc. Also, many customers use the EHCI interface for Modem Connection. If you connect a camera to this port, you can browse the pics on the SD card in camera in the gallery using the PTP daemon in ICS. Also, you can connect USB Headset or speakers and play audio over USB."
"On Blaze if you plug in a USB audio device, all audio will be routed to the device."
http://music.columbia.edu/pipermail/andraudio/2012-April/000614.html
The Google issue "Enable USB audio", requesting Google to standardize USB audio on Android smartphones/tablets, is currently ranked 11 of 22,586.
http://code.google.com/p/android/issues/detail?id=24614&sort=-stars&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars
Vote: Click on the star (it appears after signing in)