Tagging keywords, and optimizing with "tagging" in mind is really the SEO of the future. As the Internet draws closer and closer to the "semantic web", it's going to be critical that tagging is implemented.
Isn't "tagging" really just another way to identify, or carve out verticals? And in all this carving, we are helping search engines in categorizing the web, and in turn, algorithms will be derived that can start to make sense of all this clustering of tags.
Tagging is not only important, but it will, in my opinion, become the most (if it's not already) important element in SEO. I say this because not only tagging keyphrases within the content is important, but structuring your meta-data so as to be consistent with your tagged keyphrases will give any webpage a substantial amount of potential in relevance.
However, since we don't have a semantic web, it's still CRITICAL to build authoritative inbound links with proper anchor text... in short, the tagging mindset is critical, especially as we approach the age of the semantic web.
Awesome post Aaron, and clearly you're in touch with the future of the web.











