We arranged this so that it would be entirely within each user's control. SpiderOak never publishes the names to share rooms, so someone would have to know the name or URL to go there. Also, all share rooms use SSL (https) so they won't show up in proxy server logs and things like that (although they do show up in browser history).
So, if you want a search engine to index a SpiderOak share room, you just need to link to that share room on a public website somewhere. Then the search engine will know of its existence, and be able to craw/index it. If you never post the share room publicly, it will remain private.
Keep in mind, though, that your share room may be indexed inadvertently through seemingly private sharing. If you give a link to your share room to a friend, and he innocently puts that link on his blog, twitter, myspace, or similar, it will get indexed whether you like it or not. In the future we will provide more fine-grained controls over search engine indexing.