A recent post published a few minutes ago by Adam Lasnik on the same thread, regarding PageRank, internal linking and the supplemental index:
We list bazillions of pages in our main index that have no external backlinks; PageRank can flow *within* a site. While it's super to get links to internal pages from admiring fans, it's not a requirement in order for those deep pages to appear in the main index.
This, by the way, doesn't mean that you should go *hog wild* with internal crosslinking (it'll only confuse your users and fail to provide any additional ranking benefits), but it's a great idea to have a regular sitemap and/or reasonable Web of related links within your own site. That's not just good for Google, it's good for your users... because they can then (ideally) find any page/product/service on your site.