One site I visit is using mybb.
They just implemented a feature where as a guest. You can see all the topics and content, but after a while. I guess some predetermined visit count.
You can't open the topic until you register.
You can still see the forums and topic headers but you can't open them.
It works somewhat like a teaser forum here works.
The site is:
http://www.networkedmediatank.com/

Interesting idea, you've have to build in some logic so Search Engines don't get limited though, otherwise you'll be shooting yourself in the content foot