Last week at SXSW Google engineer Matt Cutts announced that Google is working on an "over-optimisation" penalty. If your site is "overly seo'd" then expect to see your rankings start to fall in the coming months.
As far as I'm concerned, this isn't really news. It's an obvious progression for Google and it's something I've warned clients about for years now: you can't afford to focus on getting users onto your site at the expense of providing relevant content to them once they arrive.
Nevertheless, this latest step by Google is easy to understand. You just need to understand what business Google is really in.
Hint: It's not the search engine business.