Authority Sites and Themed Link Building

The "authority site" is a foundational concept, like Pagerank, from which Trustrank and Favored Rank (and all other ways of valuing ranking importance/relevance) are derived.

From the Google patent, approved in 2006 but applied for in 2000, System and method for searching and recommending objects from a categorically organized information repository, it clearly states:

“Google's metric of importance is based upon two primary factors: the number of pages (elsewhere on the Web) that link to a page (i.e., "inlinks," defining the retrieved page as an "authority"), and the number of pages that the retrieved page links to (i.e., "outlinks," defining the retrieved page as a "hub"). A page's inlinks and outlinks are weighted, based on the Google-determined importance of the linked pages, resulting in an importance score for each retrieved page. The search results are presented in order of decreasing score, with the most important pages presented first. It should be noted that Google's page importance metric is based on the pattern of links on the Web as a whole, and is not limited (and at this time cannot be limited) to the preferences of a single user or group of users.”

It is important to note that this was applied for in 2000 and so many of these concepts have been further refined and these refinements addressed in numerous patents approved and applied thereafter.

Google realized that authority sites do not necessarily represent the best source of information on a particular query and continually addresses their technology to refine the algorithms. Remember, their goal is to provide semantically meaningful results to the query and give the end-user the info that they are looking for (whether asked directly or ambiguously).

It is also interesting to see that at the time of the patent above – Google did not have the technology to support more POV (point of view) searching and specific niche interest groups. Evidence of using these as we move forward in the future is apparent from the mention of approved and applied patents. These new patents will be used to enhance and refine results gleaned from human edited directories, social bookmarks, personal browser and search preferences, as well as the most recent data gleaned from the publishing of "google notebooks". 

Hopefully this gives you a good idea of where Google is going. Almost everything data-source you can imagine will be spidered and considered as part of the "Authority" algorithm used by Google to return better search results to the end-user.