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The expire framework allows for pages to be selectively expired from the cache. For forums this means that you would want parent items to be cleared when a post is posted or updated.
This is better than the internal Drupal approach which clears the whole page cache on a post.
I have written a patch for this as a stand alone module but I think it would be better to be part of the advanced forums module as if expire is not installed the hook will have no effect.