It seems to me that we have some responsibility for the licenses used on these presentations.
It also seems to me that we should structure our approach to these licenses similarly to the way we approach other license issues. That is: we should encourage people to use a DFSG license, and we should label the presentations to let people know whether it's main/contrib or non-free. We don't have to exclude non-free presentations to encourage free software. However, unlike other conference holding bodies (such as the ACM), we aren't really in the business of collecting and selling copyrighted material. So rather than asking for a transfer of license to ourselves we should be asking for a DFSG copyright on the material. But SHOULD is not MUST any more than it is SHOULD NOT or MUST NOT. You build a community by encouraging participation, not by mandating it (nor by discouraging or forbidding it). This applies to our part in the free software community as much as it applies to anyone's part in any other community. -- Raul