Like the overwhelming majority of factory or stock Android devices, with the exception of the Archos G9 tablet and the developer-targeted Texas Instruments Blaze tablet, the factory Linux-powered Logitech Squeezebox Touch does not support USB DACs.
The USB audio is enabled manually on Squeezebox Touch:
http://www.digitalaudioreview.net.au/index.php/news-blog-and-showcase/john-darkos-blog/item/150-how-to-enable-usb-audio-output-on-the-squeezebox-touch
The latest method uses an applet which allows a custom kernel to be installed and provides menus to select the audio output device.
This custom kernel can support 24-bit/192KHz USB DACs. It can also support asynchronous USB DACs which sound theoretically better than the legacy adaptive USB DACs.
http://www.digitalaudioreview.net.au/index.php/news-blog-and-showcase/john-darkos-blog/item/362-enhanced-digital-output-applet-for-squeezebox-touch
http://forums.slimdevices.com/showthread.php?94855-Triode-s-USB-24-192-plug-in-sound-quality-impressions
The USB audio is enabled on the Ainol Novo 7 Advanced tablet:
http://www.slatedroid.com/topic/29289-usb-audio-cards-support-drivers-and-libs/
"We just need 3 base things:
1st: USB Host support in Android kernel.
2nd. USB audio driver, in kernel, or compiled as a module.
3rd. Some logic to switch to USB audio card, or back to internal one."
The Google issue "Enable USB audio", requesting Google to standardize USB audio on Android smartphones/tablets, has reached the Top 9 of the Google issue rankings.
http://code.google.com/p/android/issues/list?cursor=android%3A24614&sort=-stars