See this forum discussion: http://moodle.org/mod/forum/discuss.php?d=99400 including my description of the feature request. It seems to me that the work by Dean Stringer may have already completed much of the foundation for this feature.
Something within the MOODLE framework so that users can export forum discussions on-the-fly. Example:
1. user navigates to forum
2. user reads forum, wants to archive for off-line reading later
3. user clicks some sort of "export" link that then prompts user to download formatted export file