Semantic Advertising or Semantic Targeting, which is more about targeting the relevant ad based on the meaning and sentiment of the page has gained lot of traction in last few years. I will recommend everyone to read this article by Scott Brinker which explains four different kinds of Semantic advertising in a very clear way.
In theory, the value of semantic advertising is great and companies like peer39, expert system and few others have gained lots of visibility. The opportunity is large as publishing industry is going through the biggest turmoil in decades because of this new generation who doesn't believe in paying for the content. So a relevant or highly targeted advertisement will end up being a major source of revenue for publishers.
In the end, semantic advertising is still a niche space and not a whole lot is understood by the industry about the effectiveness of this approach. There are not enough demos to show the value of semantic advertisement and publishers still need to be convinced more about the benefits of switching to semantic advertising. It will probably take some more time for semantic advertising to become completetely mainstream but I don't think that it will have any bearing on future of De Beers despite the "flaw" in ad placement - as there is a saying that "it is better to have a diamond with a flaw then to have a pebble without one." "Diamonds are forever" and they will always remain a scarce and sought after resource at least in next few decades.