Aaron Wall at SEO Book has an excellent and thought provoking write up named The Myth of a Perfectly Optimized Page. He discusses how he gets request from folks for him to review a page or two and to let the person know how "perfectly optimized" that page is. Aaron then explains how optimization is not just about keyword density or links, but that each page has a goal. Based on that goal, optimization tactics and strategies will change.
At this point, when writing this summary, I think Aaron make be discussing "optimization" as a whole, and not just search engine optimization. But when you think of it, I understood his post to apply to the 'new wave' of search engine optimization. Optimization of the page, as a whole, applies today to your search engine optimization strategy. What type of content should I write? What types of links should I get? How hard will it be for me to get natural links? You see, all these things come into play for usability, conversion testing and search engine optimization.