The user meetoblivion reported on the forums [1] that he was getting an error when he called
session.getRootNode().addNode("somepage")
and
session.getRootNode().addNode("somepage","nt:unstructured");

The full stack trace is listed on the forum thread, but the relevant part is here:
2009-09-08 10:45:10,059 WARN [org.hibernate.util.JDBCExceptionReporter] (pool-159-thread-2) SQL Error: 1400, SQLState: 23000
2009-09-08 10:45:10,108 ERROR [org.hibernate.util.JDBCExceptionReporter] (pool-159-thread-2) ORA-01400: cannot insert NULL into ("JCRPUBLIC"."DNA_NAMESPACES"."URI")
2009-09-08 10:45:10,108 WARN [org.hibernate.util.JDBCExceptionReporter] (pool-159-thread-2) SQL Error: 1400, SQLState: 23000
2009-09-08 10:45:10,108 ERROR [org.hibernate.util.JDBCExceptionReporter] (pool-159-thread-2) ORA-01400: cannot insert NULL into ("JCRPUBLIC"."DNA_NAMESPACES"."URI")

Oracle doesn't like empty VARCHAR values, and attempts instead to insert a NULL value rather than the empty VARCHAR value. This is currently a problem, because the DNA_NAMESPACES.URI column is not nullable.

[1] http://www.jboss.org/index.html?module=bb&op=viewtopic&t=160723