The sensationalistic Forbes article "Condemned To Google Hell" tries to explain Google's supplemental index, but ends up spreading more FUD (fear, uncertainty and dismay).
The article chronicles an online diamond business that suddenly lost its rankings in Google and many of its pages ended up in supplemental results:
What happened? Sanar isn't completely sure. But he does know that his site has been condemned to the supplemental index, a dreaded backwater region of Google search results that goes by another name in online marketing circles: Google Hell.
Google Hell is the worst fear of the untold numbers of companies that depend on search results to keep their business visible online. Getting stuck there means most users will never see the site, or at least many of the site's pages, when they enter certain keywords. And getting out can be next to impossible--because site operators often don't know what they did to get placed there.
Matt Cutts steps in today on his blog, to debunk the idea of Google Hell and once again explain that supplemental results are not to be feared:
It's perfectly normal for a website to have pages in our main web index and our supplemental index. If a page doesn't have enough PageRank to be included in our main web index, the supplemental results represent an additional chance for users to find that page, as opposed to Google not indexing the page.
He also does some digging into the diamond site mentioned in the story, and finds that the site has been engaged in excessive reciprocal linking, which is a possible cause for its pages to end up in the supplemental results. He offers the same advice he has given in the past: "The approach I'd recommend in that case is to use solid white-hat SEO to get high-quality links (e.g. editorially given by other sites on the basis of merit)."