I think Stack Exchange and Wikipedia are examples of SEO'd sites too. For example: Stack Exchange had problems with indexing before they added a sitemap [1]. Wikipedia takes a lot of care to create machine-indexable pages with quality content, that link back to their sources [2] and are linked to internally. Both follow SEO best practices as described in the Google Quality Guidelines [3]

Or your definition of "SEO'd" is leaning more to manipulative/sleazy.

In your example about the spectacle cases, would the other site being an affiliate make a (huge) difference? If Google punishes companies that have more than one website, people would shift to affiliates or try to hide they own the websites in other ways. In a way I feel multiple McDonalds in one city is perfectly possible. However, there needs to be a balance between the "best" websites (be they from the same owner or not) and diversity of the results (a query deserves diversity).

[1] http://www.codinghorror.com/blog/2008/10/the-importance-of-s... "We knew from the outset that Google would be a big part of our traffic, and I wanted us to rank highly in Google"

[2] http://www.mattcutts.com/blog/pagerank-sculpting/ "In the same way that Google trusts sites less when they link to spammy sites or bad neighborhoods, parts of our system encourage links to good sites."

[3] http://support.google.com/webmasters/bin/answer.py?hl=en&ans...