This is important! More and more, prospects are coming to me and I find real quick that a crisis percentage of their pages have moved into Google’s supplemental index.
The way to find out if any of your pages are in the Google supplemental (low value) index is like this:
Go to Google.com
Search Google for site:www.yourdomain.com
Start looking to see where they begin. You will know because the bottom row of data also says: “supplemental.” If you have more than 20% of your pages listed in the supplemental index then you should take action. These pages are usually stale and often share or duplicate content. If you do not know what action to take you can call me, or find an SEO expert near you to consult with.
But you should certainly at least see if you have a problem yet. 
You can also try site:www.domain.com *** -sljktf to get only the supplemental results
There is a new, related post, of equal importance here:
http://www.mrwebguru.com/blog/search-engine-optimization/bye-bye-google-supplemental-results/