[Ted S]

Originally Posted by FleaBag

I have a fear if Google realised your page was using this it could really damage your ranking.

That's certainly possible. However, many sites (i.e. webmaser world, new york times, barons, wsj, etc...) do just this... and do it with Google's knowledge. In fact, Google has a product that logs in to protected sites and from what I've been told, will be rolling this out to more sites in the future.

Keep in mind that you are not showing different content to a user any more than Amazon shows people who have logged in different content than those that have not. Most dynamic sites these days show different content to different groups -- the welcome message on your forumhome template, showing birthdays or announcements to one group, or ads to another. In this case, all you are doing is limiting the number of times a guest may view pages. I can't tell you what this will or won't do for the SERPs, I can tell you why it makes sense and what others are doing that's similar.