From the article: "Since the Panda update, more and more people are trying to control their Google index and prune out low-quality pages.
I’m a firm believer in aggressively managing your own index, but it’s not always easy, and I’m seeing a couple of common mistakes pop up.
One mistake is thinking that to de-index a page, you should block the crawl paths. Makes sense, right? If you don’t want a page indexed, why would you want it crawled?
Unfortunately, while it sounds logical, it’s also completely wrong."
Full article: http://www.seomoz.org/blog/logic-meet-google-crawling-to-deindex ;
Via Robin Good