This article by Erick Schonfeld believes that if the Semantic Web becomes the commonly accepted standard it is likely that websites will have to be rewritten to accommodate this:

“Semantic search is perhaps the most promising, but it essentially requires every single Webpage to be re-written.”

For the full article see: http://www.techcrunch.com/2008/04/25/is-keyword-search-about-to-hit-its-breaking-point/