It takes time for individuals to find the best ways to manage their own participation. I'm still working on that with Google Wave! And even though I'm not too keen on filling up my email inbox with more messages I've found the wave notifications (sample attached) to be quite useful.
It seems a mighty important first step in using Google Wave in teaching would be to implement an activity that gives students an opportunity to try out various strategies for managing their participation. It's part of the "Getting students to join!" practicality that Emma mentions.