It looks like there is a bug on line 74 of org.hibernate.exception.ErrorCodeConverter in hibernate-3.0 final (and CVS HEAD):

return new ConstraintViolationException( message, sqlException, constraintName, sql );

Should be:

return new ConstraintViolationException( message, sqlException, sql, constraintName );

i.e. the last two constructor parameters are in the wrong order.

Fixed in my box, thanks.