I used to have DUN working, via an invocation of dund like this:

/usr/bin/dund --listen --persist --msdun call centro

This no longer works (I hadn't tried it in a while, and it has not worked for
the last few days).  Syslog shows lines like these:

Jul 14 23:40:38 localhost bluetoothd[15478]: link_key_request (sba=00:19:86:00:2C:40, dba=00:07:E0:E7:25:6E)
Jul 14 23:40:38 localhost bluetoothd[15478]: DUN client disconnected while waiting for authorization

And the connection (from a PalmOS device) eventually just times out.