This plugin does the following:
  1. Adds 2 tables to the database - one to store sitewide topics and another to track blog/topic relationships.
  2. Defaults all blogs to topic id 1 (defaulted to "uncategorized")
  3. Adds a topic chooser to the blog registration form.
  4. Adds a sub-menu to the site admin screen to allow adding/editing/deleting of sitewide topics.
  5. Adds a sub-menu to the blog settings admin screen to allow blog administrators to change the topic (but not add or edit site-wide topics).
In addition, a 3-in-1 widget is included to display current topic, related blogs, and related posts. Code available here: http://wordpress.org/extend/plugins/blog-topics/Â