Today, I logged into my Facebook account and was greeted by an announcement that Facebook is intending to make it possible for non-members to search the directory and for the directory to be indexed by search engines.
There's more info at the Facebook official blog.
You can change your settings to opt out from all of it. Or, if you're on Facebook to be found by others, you can make it available. I have to give Facebook credit for making it obvious to users that they are planning to do and giving a full month to decide what they want to do.
For other thoughts on the topic, you can check out the discussion over at Compiler - Wired Blogs.