It could also say underneath something like "As a guest, you can only read the first post. To view the rest of this thread or replies, you must log-in or register." (With links to do that). The text should be editable in the admin panel and be able to be changed.
The other benefit is the SEO and traffic development. Now you can have juicy content in the search engines and still keep your forum site "private". Would also be great to use on paid membership sites that use Xenforo as a platform.
Anyone else interested?