After some speculation, Google has announced that it would be removing the Supplemental Results from the search index. However, this only relates to the tagging of these results - noted in the green URL/information line below the search listing - rather than the pages themselves.

These Supplemental Results have existed for many years and represent pages that are seen as less important by Google's indexing criteria, although they can still be displayed within quite specific search results. However, lately the number of supplemental results have become an issue with web marketers, who can identify the pages and then make changes to try to re-establish them within the the main indexes. At the moment, the method of displaying these pages still works, although this may also disappear.

So Google is now 'hiding' the pages that are identified as Supplemental, although the categorisation and use of these pages will still remain. It will just become harder for website owners to identify these, although the basic principles of avoiding pages becoming Supplemental will remain - unique fresh content and a good link structure being the primary considerations.