And here's a wild conjecture: That could have a knock-on effect, where pages that had a lot of backlinks from supplementals lose their backlinks, and therefore themselves become supplemental- causing pages that they link to to lose links and so on. If that were true, you would see massive upheaval in some parts, and in other parts blue skies, a light breeze, and plain sailing.
That's not wild conjecture Callivert; what you describe is exactly what I'm seeing. Months and months of slow boiling. Google has been cooking the "unimportant" pages out of their main index into the supplemental index.
This is proving very effective in taking out entire categories and neighborhoods of links Google doesn't want influencing their results: huge scraper sites, forum and blog comment spam, reciprocal links, crappy directories, etc. Most all have been boiled off supplemental. The result is a fundamental shift in Google's distribution of link popularity across the web.
Unfortunately this has also sent many important pages for obscure queries supplemental. Users can no longer count on Google consistently returning good results for obscure queries.
A friend of mine was extremely frustrated when she couldn't find a page a second time for a query: "STATENAME fingerprint card supply hours" It gave her the exact page with the hours of operation as the number one result the first time she tried. Magic! Two weeks later she needed another card after her prints for her bar application were smudged. The page was nowhere to be found and she was very frustrated. I did a bit of investigation and the result she was looking for was on page 3 and had gone supplemental. Yahoo and MSN couldn't find it either, but it was a step back for Google.
I ABSOLUTELY agree with this theory! In fact I have described it previously in threads.
I referrred to it as the "Tsunami effect". This would explain why month after month "new sites" are 'hit' with the same symptoms as the ones that were 'hit' in x (pick any month)serp changes...
This is spreading like a huge wave. Next month - another batch of 'new sites' will be signing on expressing the same list of wows - as did the sites last Sept, Oct, Nov, etc.
THE GOOD NEWS IS... IF this is truely the cause of sites going MIA - then a good solid link campaign should help bring your site back to some stability in the serps.