Here's the basic scenario I'm looking at: Someone comes to the website (which features various journal and news articles), and can look at a certain number of articles "for free" for a particular length of time (say 30 days). However, once they hit that number of articles within the length of time, the site displays a registration form instead of any articles.
Obviously, if someone is a registered member of the site and logged in to their account, they have unrestricted access and can read as many articles as they want.
It's a readership/subscription model similar to the one employed by http://dish.andrewsullivan.com/, FWIW.
I can envision how this all works -- it's a pretty straightforward membership/registration set-up -- except for the part about tracking/counting the number of articles that someone who isn't logged in has read, and then doing something once they hit a pre-defined limit.