This version fixes a bug with the creation of new topics and fixes the error handling in admin/user.php. It also includes updated language files and a sample implementation of the CUSTOM_mail function (for those having problems with the PEAR::Mail class).
We expect this to be the last Release Candidate and the final 1.3.9 could be out in a week or so ...
When upgrading from 1.3.9rc2, you only need to replace a few files (see docs/changed-files for a complete list) - there were no database or theme changes.