A site with no links to it at all, as Google themselves say on their own site, wouldn't be in the index at all. If it did happen, presumably its PR would be zero.
Again, a page with no links to it wouldn't be in Google -- it's an orphaned page. PageRank "flows" not only from outside links, but also through your internal link structure. So your example page might have a PR of 5 because it's linked to from another of your pages with a higher PR. For PageRank calculation purposes, there's no real difference between a link from a page within the same site and a link from an outside page.
Vitaplease, that was exactly my point. The original question was specifically about PageRank, and presumably about PageRank as it's calculated today. :)
But really I agree with dstanovic; focusing too much on building PR to the neglect of just getting good links and performing strong on-page optimization is failing to see the forest for the trees. And in the end will probably be beaten in actual serp positioning by those who concentrated instead on those two techniques.