Yes, but i see many vary famous forums that using it around the years...never happened to see one of that penalized or banned by google.
In theory you're right, in the pratice i think that Google (the quality raters, i don't think that a bot can recognize this "trick) are sufficiently intelligent for see that it's not a trick for have more google visits. IMHO...i think it's a grey hat tecnique. But i'm love to see a mod to doing that on Xenforo. Very very useful to attract new users.
It works by putting a cookie on the users browser, should they read x amount of posts the cookie stops them from viewing any more. Guests could clear cookies to bypass it but a lot of them don't know that or just are to lazy so they have to register/login.