Rufus_dog - 7:55 am on Jun 25, 2007 (gmt 0) Anyway, these are just my observations.
When Google first developed the supplemental index, pages went to supplemental results because of duplicate content (within site), lack of unique title and description, etc. But later on, they started to put more and more inner pages to supplemental result. Even if your homepage has lot of backlinks, and the PR is transferred to first level inner pages, these inner pages can still be put in supplemental results if they don't have any sites linking to them directly. The higher your website PR, the more pages Google will let you in the main index. If you have a new site with few backlinks, your homepage can easily get a PR3 but even with original content, most of your inner pages will be in supplemental results. If there are two websites, both have 200 pages. The one with PR6 can have more pages in the main index than the other one with PR3.
Anyway, these are just my observations.