I am definetly missing something
I have just run the same on the system it worked before and got the same result

1. login as sysem in db and run sql - select count(*) from dba_objects a, dba_objects b; just to run for a while
2. login as sysdba from different session
3. Get SID and SERIAL#
4. EXECUTE SYS.dbms_system.set_sql_trace_in_session (14, 470, TRUE);
PL/SQL procedure successfully completed.
5. Checked udump dir .. -nothing there