MediaPost: As marketers grapple with jargon like keyword density and struggle to devise bidding strategies, many may not be aware that some paid search results are being driven by user behavior and perceived relevance as opposed to strict keyword matching. Overture, Gator, and Applied Semantics are all playing the game where by they propigate the search with terms that may not be exactly what you typed, but what the search provider views as "conceptually relevant categories" that may interest you.
http://www.mediapost.com/dtls_dsp_news.cfm?newsId=193791