Why on earth are you using 9.2.0.1? With all due respect you should be patching your databases. This works fine in 9.2.0.6.
Code:
Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
JServer Release 9.2.0.6.0 - Production

SQL> SELECT CURRENT_TIMESTAMP - TRUNC (CURRENT_TIMESTAMP)
  2  FROM   dual;

CURRENT_TIMESTAMP-TRUNC(CURRENT_TIMESTAMP)
---------------------------------------------------------------------------
+000000000 09:48:51.709693

SQL>
Good suggestion regarding CAST but I don't think that is going to work in PL/SQL.