The ability to approve forum discussions/posts obviously did not make it in to 1.5 but there seems to be a renewed interest in allowing teachers to approve posts (http://moodle.org/mod/forum/discuss.php?d=104476) which seems like a reasonable request considering we have such mechanisms in place for database and glossary. Therefore, I'm going to edit this and make it a feature request for 2.0.
In terms of implementation, I'm thinking it would be good to mirror the database module and add an approval field to the forum table, and approved fields to the forum_discussions and forum_posts tables. The rest would just be a matter of designing the UI. For the forum approvals, perhaps we could add it to the table on /mod/forum/view.php. Perhaps we could add a capability for forum approval (of discussions and posts).
On the /mod/forum/view.php we could add a Needing approval link that would display all of the posts needing approval perhaps with check boxes on the side and a select all that would allow the approval of several or all at once resulting in a table with the following columns: Discussion, Started by, Replies, Needing approval, Last post. Then on /mod/forum/index.php add a pending/approval column to produce a table with columns of: Forum, Description, Discussions, Subscribed, Pending (or approval). With the database module the index.php file does not link to the ones needing approval it simply indicates that they exist allowing links to the database. Similarly, the teacher could click on the link to the forum and then click on the link to the one needing approval from the view.php page.
Just a few thoughts. Feel free to kick around other ideas. Raise questions, etc. If I manage to find some time I would be willing to start hacking on this. Let me know if this is something desirable and if my ideas sound reasonable.
Peace - Anthony