While it is appropriate to have the PropelPDO subclass override commit() so that nested commit() calls are ignored (see http://pecl.php.net/bugs/bug.php?id=7718 for why we do this), it is *not* appropriate to do this for rollBack() statements. The current implementation will result in nested rollBack() statements being ignored. Only a rollBack() statement at the top-level will actually take effect
