As of today, October 16 2008, everyone with a user account on Drupal.org is able to edit most handbook pages (details are noted below). All users can already create new pages in the handbooks so this expands on that to allow editing of pages other than those you created. In the past this has been permitted only for members of the documentation team or site maintainers.
This new permission is currently set to last for a trial period of one month. At the end of the period, on November 15 2008, the documentation team and site maintainers will assess if the trial has been successful and make a decision whether to continue to allow open editing. If the change causes too much spam or vandalism in the handbook, then we will be forced to revoke open editing. If we are unsure, we may decide to extend the trial period in one month increments until we can firmly make a decision one way or the other.