The term clusters has become the newest SEO jargon word to be bandied about. Clustering related search terms into groups has already been implemented by several smaller search engines such as Vivisimo's Clusty, but the other major search engines (including Google) will likely soon follow. Keep this term in mind as you optimize your site. It may be something that you will need in the near future. Check out the article at http://www.searchenginelowdown.com/2004/10/web-20-exclusive-demonstration-of.html for some interesting information about how Google intends to use clustering in the future.