Dave: You have already received a lot of good comments on your question. I would only add that I have on occasion, put the theme up on an easel in front of the group, and asked, "Is this the theme?" In a couple of cases they said "no," and preceded to refine the theme until it reflected what the group really wanted. I found that this was important as a first step in that it gave ownership for the os event to the participants. Otherwise, it might have been "my open space," which wouldn't have worked.
Dave Koehler