Limit the number of posts a category can have – newest post goes in, oldest one drops out, possible? plugin?

by

  1. Hook into publish_post.
  2. Check if the post is in the category news.

  3. Get all posts with that category: 

    $query = new WP_Query( 
    array(
        'category_name'  => 'news', 
        'posts_per_page' => -1 
    )
);
  4. Update the category of the oldest post if necessary: wp_set_post_terms().