My situation was similar to frankleeceo except that it was not a wordpress blog but a static html site. Somehow Google and other bots created 1,000,000 rubbish URLs out of my 1600 pages that my server recognized as valid. My "not selected" line spiked and my rankings dropped. I wouldn't say they tanked. Once the htaccess file was reconfigured to make the server return 410s, the "not selected" line stopped rising. Now, it's dropping by about 10 pages a week. Rankings have still not returned.