@Estel : no, not USB networking, rather what you call "client DAC" -- see examples below

@pali : this is astounding. Does it really answer the question in the thread Oblomow pointed to ?
Meaning, sending out digital audio signal through N900's USB port to an external device ? Without USB
host support ?

@Oblomow : yes, that is the idea, although my use case is a little different.
My idea would be to re-purpose my N900 as an "audiophile-grade" portable music player. I would use its internal 32GB (+SD card) for MP3/FLAC storage, its CPU/touchscreen for the UI (probably Rockbox), but not its audio system (internal DAC and amplifier).

The challenge is to get the decoded digital audio stream out through the USB port, and feed that to a portable DAC+amp combo such as one of these :

http://www.fiio.com.cn/product/index...&MenuID=020301
http://www.fiio.com.cn/product/index...&MenuID=020301
http://www.ibasso.com/en/products/show.asp?ID=73
(there are many more :-)

These gizmos behave like external USB sound cards, so I'm pretty sure USB Host is needed. On a PC with Windows or Ubuntu, you just plug them in and the sound comes out...