In response to the interesting issues raised by Bronwyn: I agree that there is a need to design activities for 3-D MUVEs that utilise the affordnaces that a 3-D MUVE provides.
I have been in a few conferences in Second life in the last few weeks (e.g. SoodleMOOT, SL Literary festival) and I agree that if its just the delivery of a lecture - there are other ways such as podcasts, web-seminars or pre-captured video recording in Adobe Captivate, and one doesn't necessarily have to be in SL for such lectures. However, its the synchronous interactions that follow a live event in SL that really add to the experience: Q&A session with the speaker, the visual and verbal intearctions with others in the audience, and getting to network with others attending the event. I have stayed behind (after the session was over) in some of the sessions just to talk to other avatars who were raising interesting issues with the speaker and have in this way encountered colleagues from other part of the world with similar research interests.