Interesting... many thanks.
The next job is to find out if the Nexus 7 connects to some phone (that definitely supports the DUN USB profile, e.g. a Nokia 6310i) over bluetooth, and whether it offers the DUN option when the bluetooth device is right-clicked on.
Interesting that Android appears to have deliberately clobbered the tethering option. Are google looking to enter into deals with big telcos?