After getting a weird NullPointerException using Transaction.commit() in Hibernate during an insert (eventually calling PreparedStatement.clearParameters() which was the source of the exception), it turned out that the JDBC driver in classes12.jar is outdated, and will not work on JDK1.4 and an Oracle 10g database.
Download the new drivers here to prevent unsuspected errors such as this one..
For a some interesting samples of what you can do with this new driver check out
this link on OTN