Actually, the two caching paradigms I've been thinking of looking into might be called "cache-interval" and "cache-key".
The former is similar to what Robert's talked about above.
The latter might be used for sets of pages that are closely coupled, for instance those that display forum and message information in the bulletin board. These are all dependent on a single key - the forum_id and there's no reason why message add, edit and delete pages couldn't essentially say "kill all cached pages dependent on this key". This would require no db access. It would take real care in the implementation of such closely coupled pages but the payoff could be very big. Caching a forum summary page until someone makes a new post, for instance ... on a site like this that is slowly growing a large number of posts yet only collects a few dozen posts a day cached copies would have a significant lifetime.
