Google tells webmasters that backlinks can also influence adversary to pagerank, so the most important issue is to get backlinked from relevant sites(or sites with content relevant to yours). Wikipedia states that PR is measured due to quality, quantity and strength of backlinks( or inbound links to your site). So all mentioned above (in the post) are true and should be considered. So you must not get backlinked from sites with PR 0. Better concentrate on your site's content quality. This is for other search engines, which does not include backlinking for search result ranking but quality content.
Only one way, content shall stay king and if that happens, rest assured, people will link you without you requesting them to do so.
Yes that is good saying. I missed that.I would call that natural backlinking. I used to learn webmaster programming through w3schools, and I really liked that site so I often wrote about it. That would be a natural backlink for your site.