Albaengine

Theming & semantic connectivity have become big issues in search technology. Google & Yahoo! have each purchased smaller companies whose focus is on the relationships between words & concepts. In the modern algorithms, relatedness plays a large role when determining which inbound and outbound links are valuable in ranking a site.
Three primary factors of search engine rankings are presumed to be influenced by topic-sensitivity - PR, links & site/page theme. PageRank is supposedly a measure of the general importance of a page, but many speculate that PR is now being passed and measured based on an analysis of relevance to the general or specific subject matter. The value of a specific link and the theme of a site/page is also estimated to be calculated based on the relativity of the subject matter.

The best evidence for a themed set of factors is Google's development of the "Site-Flavored Search". This unique tool enables site owners to select the theme that best fits their website, then offer Google's search functions slanted towards their specific industry/market. At the very least, the existence of this technology indicates that Google is using a system of recognition to theme websites into groups. An extreme use of this technology would allow Google to discount irrelevant links from "un-themed" sites, thereby reducing the value of non-targeted link building. Google could also use this technology to categorize websites and eliminate non-relevant pages from the SERPs. Clearly, this isn't taking place yet...
Topic-sensitivity, however, is something that needs to be planned for. Prior knowledge of this phenomenon should make it clear that by using Google's own flavoring system, SEOs can pick out logically which websites will provide the greatest relevance and links. This filtering does bring up some important points which are still open to debate:

	Could search engines in the future filter out directories or non-targeted sites as being
	less relevant?
	In planning for the growing dominance of topic-sensitivity, should sites be targeted at
	single, specific subjects?
	When creating pages for current websites, should content remain segmented to a strict
	interpretation of the website's primary topic, or is it OK to branch out?

For all of these questions, I would predict that search engines will not swing the pendulum too far, or they risk severly crippling current results. The penalties or devluations for even large swings away from primary topics will probably never become a big factor in SEO. The goal of the search engines is to provide relevancy, and damaging thousands of sites who don't fit a rigid structure is very unlikely.

That said, there are many steps that should be taken in order to benefit as much as possible from growth in topic-sensitivity. A list of important and somewhat obvious steps follows:

1.	Use Google's site flavored search to look for links using searches like intitle:"submit url"
	OR "add url", etc.
2.	When considering the purchase of links, check to see if the site fits into your specific
	niche, or at least the category one level above you.
3.	Search for your primary keyword phrases and see what category/categories the top 20-
	30 results fit into. This can give you a good idea of what the current usage of topic-
	sensitivity is for your terms.
4.	Google's site-flavored search can also tell you who is "themed" in your category - try
	conducting a wide variety of keyword searches after customizing a search to your theme
	and see who is listed with the Google "bubble" logo next to their link - these sites have
	been "themed" in Google's index.

The last subject is PageRank, which is becoming a less critical piece in Google's algorithm, with probably credit going to increased theming. Topic sensitive sites are less likely to have the exceptionally high PageRank of the Internet industry's giants, but these pages are more 'relevant' to the search query. However, for sites whose business model may rely on PR, or whose competition is so fierce that high PageRank is neccessary, it may be important to note that some SEOs have predicted that PR will no longer represent 'absolute' importance in the future, but may use some measure of theming to make the calculations. Luckily, the advice above still holds - continue to link build from important sites that are in your category and you can't go wrong.