I've run into the same problem (no ringtone, phone on speaker, no DNS, mediaplayer and camera stall because they can't use audio). A few weeks ago I set up USB networking and added usb0 to /etc/network/interfaces, as described in http://wiki.maemo.org/N900_USB_networking , and after restarting my N900 (after a flight) it was in this weird state.

Now, I fixed this by returning /etc/network/interfaces to its original state. The weirdest part is, though, that just commenting out the lines pertaining to usb0 didn't do the trick, only deleting them altogether does.

Any comments / explanations? It looks to me that using USB networking requires keeping two versions of /etc/network/interfaces, one without the usb0 entries, to be used during startup, and the other to be used for USB tethering...