For the forums side, yes. We've been phpBB2 (long time) > phpBB3 (short time) > Drupal Advanced Forum > phpBB3. On the "content" side, we previously had a mix of static html/php and Mediawiki. That has all been migrated to Drupal as well. We didn't go back on that part, just the forums.
At one point during the first phpBB3 phase, I had tried Forum Runner, an alternative to Tapatalk. I'm not opposed at all to integrating Tapatalk. Hopefully it could play nice with the phpBB > Drupal integration.
Tapatalk also offers a service for custom-branded app.
http://tapatalk.com/build.php That might be cool. Only problem is the $29 (iOS or Android) or $39 (both) monthly fee. That might be worth it though. Mobile traffic is currently 4.7% of total traffic. That is up from 2.9% last year. A mobile ap would probably help that.
The custom apps do offer ad support, which could help offset the cost. Would anyone be offended by that?
Edit - Looks like it will work with Drupal+phpBB3. This is for D7, but I don't see why it wouldn't work with D6.
http://support.tapatalk.com/threads/inc ... post-69559