At Facebook, we're always striving to make a more open environment for
developers while respecting our users' privacy. So far, we've enforced
this by requiring users to sign up for Facebook before being able to interact with an application. Unfortunately, this means applications are inaccessible to the millions of people on the Internet that haven't joined Facebook yet. This policy also prevents applications from being indexed by search engines that might fuel their growth. Clearly, this is not ideal for developers.
As of tonight, 12/4/07, this will no longer be a problem. We will start allowing people to access application canvas pages even when not logged in to Facebook. Of course, we're concerned about our users' privacy, and so the only user-specific data available on public canvas pages will be first name and profile picture (and then only if the user's profile picture is already publicly searchable). But you, the application developer, need not worry; FBML tags will automatically handle privacy rules for you.
The technical details of making your pages publicly accessible are on the wiki at http://wiki.developers.facebook.com/index.php/Public_Canvas_Pages.