There is an excellent Cre8asite Forums thread brewing on the topic of Google's Supplemental Index.
The thread basically asks if the percent of pages on a specific domain can be used to determine the overall site's importance in the eyes of Google.
Barry Welford said:
All we're trying to find out for a website is whether in Google's eyes the content is valued as well as it should be.
It would be nice to have something a little more informative than the Toolbar PageRank measure.
JohnMu, a new member to the Google team, but an old face to the SEO forums, is in that thread trying to give us insight into that question.
It is clear that pages in the supplemental index are not as important as pages in the main index. The big question is, how reliable is that index now that Google hide it from us and now that PageRank is all over the place?
I suspect a thread like this will continue to evolve and maybe we can learn a thing or two from it.
Forum discussion at Cre8asite Forums.