From a cognitive science perspective, the semantic web is still years and years and years away--at least a full decade. What I mean is the set of complex mechanisms that involve creating meaning, not the usual ridicule hyperbole out there.
Consider, for example the fact that when the TAM flight crashed in SP last July, the news were full of contextual ads suggesting readers to "Fly TAM".
Or maybe take a look of these contextual ads (hat tip to Digg):
The best ideas over this issue are by Bob French--though he doesn't particularly address 'contextual ads'--but the whole problem of meaning extraction from text databases in which semantic web engineers are falling upon. This paper is one of the funniest, and most intelligent, things I've ever read.
The "long tail" will always be algorithmic. The "fat head" will always be mainstream. The "middle ground" will be social. This naturally suggest a strategy for Yahoo! (which TechCrunch says is failing--and it just might be).
Yahoo! isn't mainstream media, nor algorithmic (like Google). From this point of view, I think what they should do becomes clear: They should strive to dominate the middle space.
Yahoo! should go beyond del.icio.us and acquire digg. It should subordinate all of its strategy to having all content, including ads, brought up by social voting. If an ad is buried, let it go; just like every piece of content. In the short-run, most likely, only ads from Apple or Ron Paul will appear; in the long-run, only good, socially targeted content should arise.
Meanwhile, algorithmic contextual ads will keep suggesting to stone people to death, to find debt, and to burn babies.