Lab_Rat - 7:35 pm on Dec 3, 2012 (gmt 0)
A couple weeks ago, we removed a big chunk of URLs from the site. Since the removal, the number of "Not Selected" URLs has plummeted from about 15,000,000 to about 1,000,000. I'm not certain what it was on those pages that was causing the issue, but I know those pages were the source.
We have not seen any change in our organic traffic since, but this change was made the day of the last Panda refresh. Fingers crossed.