Dimdim does not currently retain any content that is uploaded or shared during a meeting. This means that all chat messages, audio or video streams, shared presentations which are used are purged at the end of the hosted meeting. Further, no one can join your meeting or view your shared content without your knowledge since they require your unique meeting key. Finally, all meeting attendees are indicated in the participants list. It is possible to restrict entry to your meeting room by enabling the Waiting Room.
If you plan to deploy your own onsite Dimdim server you can secure it behind https and firewalls.

With Dimdim 4.0 we've implemented a new meeting key feature that requires the attendee to enter both the meeting room name and the meeting key (aka password)