Code freeze begins September 1st. During this period, we fix bugs and when Drupal 7 is stable, we release. In the past, we have done manual testing. This time we have an automated testing framework and already a large number of tests. In an ideal world, we would only release when all code is touched by tests. While this won't guarantee that Drupal will be bug free, every test contributes to the overall quality of Drupal.