I could have sworn I had drilled into my head (from studying for the Oracle 10g OCA exam) that DDL is audited in the alert log. I had a need for this today but it wasn't showing up.
Turns out, DDL in the alert log is a new feature in 11g, and to enable it you'll want to run:
ALTER SYSTEM SET enable_ddl_logging=TRUE SCOPE=BOTH;
For older versions you'll have to audit another way. One way that worked for my purposes today is a simple DDL trigger: http://www.dba-oracle.com/t_ddl_triggers.htm