
We're working on search for discussions. In the meantime, here's a trick you can use. Go to Google and search for site:http://www.geni.com/discussions and your search term. This will search all but the most recent public discussion posts.
For example, if you were searching for discussions that mention "Mayflower", you should enter "site:http://www.geni.com/discussions Mayflower" in the Google search window.