Maybe it's best not to ask what happened until we understand what it actually means. A supplemental result is one which is not in Google's primary index, but rather a secondary database which contains pages which the search engine have identified as 'less important' - measured by their PageRank algorithm.  If you looked closely at the pages of your site that were indexed in Google, you would know at a glance if an individual page was noted as supplemental because it had the "supplemental" tag associated at the bottom of the description (next to Cached and Similar Pages, etc.). This gave those optimizing their sites an indicator that they better start building links to that page, clear up any duplicate content issues and in short, freshen up the page.

As of Tuesday July 31st however, supplemental results are no more. That's not exactly accurate - supplemental results still exist, but now you won't be able to tell which are in the primary index and which are in the supplementals on the search results pages - and some SEO's are pretty upset - but not necessarily for any good reason.

Google is essentially saying that they are making this change to improve the search experience for their users and has overhauled the way in which their system crawls and correspondingly indexes supplemental results to be deeper and more continuous. The result should be that supplemental results will be fresher and more comprehensive. Via the blog post, "We're also working towards showing more Supplemental Results by ensuring that every query is able to search the supplemental index, and expect to roll this out over the course of the summer."

There has been a lot of chatter about the issue this week and being the Google watcher that I am, I expect (or rather hope for the sake of the thirty or so commenter's on that post) that Google will make some concessions and include some sort of reporting about which pages are in the supplemental index and which are in the regular index. If not, that would certainly be a wonderful feature to have available.