Steps to reproduce:
1. Set defaultfrontpageroleid to guest in Administration > Front Page > Front Page settings
2. Logout then login again as admin
3. Attempt to post in site news forum
The admin is able to edit and delete existing posts in the site news forum, but the "Add a new discussion topic" button is missing.
This problem is also reported in a forum: http://moodle.org/mod/forum/discuss.php?d=97531