Ning2phpbb has the following features:
- it imports users with avatars
- it imports posts
- it can rewrite internal links
- it is bundled with a script that generate new passwords for all your user base.
But it is really just a hack, so use it at your own risk !
PhpBB is perhaps not your platform of choice but it is quite versatile, Once your data is in a phpBB database, it should be easy to export it to another platform. Do not hesitate to modify these scripts to fit your needs, we hope this can help you importing your ning data in phpbb...