cmn_err output always goes via OBP on sheffield (and other similar systems).
Systems which have standard in and out set to the same device and without keyboards will never use the console driver stream.
I presume this is a bug and that it was always the intention for cmn_err to use the console stream (when possible) on such systems.
Work Around
If its a problem set cn_conf/W0 in kadb after the console is set up.