hannamyluv - 4:33 pm on Apr 26, 2010 (gmt 0)
I have been seeing this for close to two years now, though I am now seeing it on far more sites. I even asked about it awhile back, about the possibility of a hidden index.
I suspect that G has brought back a supplemental index, like old school. I refer to it as the sub-supplemental. There, but not called on unless they really, really need to, like when you do a site:example.com keyword search.
I see it most with sites that have incomplete sitemap.xml. About 60% of the time, creating a complete sitemap pulls the URLs out of what I suspect is a sub-supplemental index.
In almost all cases, pages in this area are weakly linked, low content.
I guess, what I am trying to say is this is not new and I doubt it is going away.