Yeah, I writing this article about why paid-archives don't work for online publications....
   http://celebritycola.blogspot.com/2005/01/why-print-pubs-shouldnt-charge-for-old.html

.... and I knew that Google was now returning these "supplemental results" that were usually links to subscriber-only content. The interesting thing is that it seems Google is crawling and storing search info for pages that are blocked from non-subscribers.... then, Google returns what appears to be a good search result, complete with a snippet of relevant text from the web page and the page's title and address, but when the page is visited it tries to get you to subscribe. Also, no "Cached Page" is available for this sort of Google result. I've known about this for a long time and I knew that it was growing – the NY Times is looking into something similar. But I'd never learned the specifics – how does this system work? Is Google collecting revenue from this? Are they actually slipping in paid search results in this way? etc.... But after a long search, the only relevant pages I've found on the topic so far are along the lines of "hmmm... I jsut noticed this supplemental Google thing, I wonder...", so I'm leaving this comment here in order to continue the discussion...

For an example of a Google "supplemental result," search for anything involving Variety.com... try, for instance, "George Clooney Variety.com" and you'll see what I mean.

Here's what Google has to say on the topic:

"Your pages may be displayed on the Google search results through our supplemental index. Google currently does not syndicate certain beta features, including the supplemental results index. Supplemental results are triggered on a relatively small number of queries for which Google's main index does not provide many results. Because this index is still in testing, we do not feel that it is ready to be offered to our Adsense for Search partners."
https://www.google.com/support/adsense/bin/answer.py?answer=10526&topic=167

This was what's on the Google search results help page at http://www.google.com/help/interpret.html – it has been removed, so below you find the result from the Wayback machine:

"Supplemental Result
Google augments results for difficult queries by searching a supplemental collection of web pages. Results from this index are marked in green as 'Supplemental.'"
http://web.archive.org/web/20040202015509/http://www.google.com/help/interpret.html

Not by Google:

"Another curiosity emerged in August 2003, two months after my "Google is broken" essay. Google started showing supplemental results from an entirely separate index. If you run out of regular results you will often see the label "Supplemental Result" in green on the last page of available links. At that time Google briefly stated on their site that they "augment results for difficult queries by searching a supplemental collection of web pages." A representative from Google had little to add to this, but did concede that it is an entirely separate index, and then threw out a few words of spin. It sounded like a cover story. I believe that this new index was started due to a capacity problem in the main index and the need to develop new software."
http://groups-beta.google.com/group/misc.activism.progressive/browse_thread/thread/a4281bc9a9ca45f7/732414dd8538ff68?q=google+%22supplemental+result%22&_done=%2Fgroups%3Fq%3Dgoogle+%22supplemental+result%22%26qt_s%3DSearch+Groups%26&_doneTitle=Back+to+Search&&d#732414dd8538ff68

This question came up some time ago in German seo-board Abakus. In German the Supplemental Results are called "Zusätzliches Ergebnis".

I tried to answer it in my SuchmaschinenWiki:
http://www.suchmaschinenwiki.de/Zusaetzliches-Ergebnis
3 quotes from GoogleGuy and an attempt to explain it.

Just after writing this last post, I tried searching for "George Clooney Variety.com" and I wasn't able to duplicate the results I had the other day....

Quote the Googleguy from Jan's Wiki:

"1. Supplemental results are results that we can show for some more arcane queries that typically have fewer results. Danny Sullivan had a good write-up about them a little while ago: http://searchenginewatch.com/searchday/article.php/3071371
It is a different index, so some of the results can be a little older, but some of the supplemental results are pretty up-to-date too.

2. Hey, the supplemental results are a new experimental feature to augment the results for obscure queries. This is a new technology that can return more results for queries that for example have a small number of results. So it might not affect the results for a popular search, but for a researcher doing a more specific query, it can improve the recall of the results. The supplemental collection of pages has been collected from the web just like the 3.3 billion pages in Google's main index.

3. Hey, pages get added to the supplemental index using automatic algorithms. You can imagine a lot of useful criteria, including that we saw a url during the main crawl but didn't have a have a chance to crawl it when we first saw it."