The friction involved in charging the battery and unplugging the USB Audio interface when more power is needed, makes USB+Digital Audio a non-starter for me. Once I've got the N900 in-line doing audio tasks, I don't want to think about charging the unit or interrupting my audio work!

For me that feeling extends to many other potential uses of N900 USB (DVB, mounting an external disk). In fact, if typical USB accessories had better ways to supply power to upstream mobile devices, we might start seeing mobile devices become more involved in studio work. Sadly for now it's a very niche thing.

EDIT: Super happy to be proven wrong