Google ranks pages according to many algorithms, the most important factor being incoming links. Whilst Google Page Rank does not directly affect your SERP’s, the back links from trusted sources do. The way this works is that Google looks at the PR of the referring web site and passes on PR. The influence of this “bleeding” affect is determined by:

• The PR of the referring site
• The number of outbound links on the page containing your back link
• The “trust” rating of the referring web site, according to Google, which is based on the registration date and consistent content, as well as the web sites own back links and these same parameters

This, put in basic terms, means that spending the time that it takes to obtain a back link from a site that has no PR is meaningless.
Here is an example of Google’s “weightedness” (a made up word by Neil);
Site 1 with a PR5 has 50 links (the max you want on 1 page) = bleeds .0012 PR
Site 2 with a PR5 has 10 links = bleeds .430 PR
Site 3 with a PR5 has 2 links = bleeds .776 PR
Additionally, Google seemingly awards back links from .org’s slightly higher, and back links from .edu’s and ‘gov’s significantly higher. This opens many vertical possibilities when taken into consideration whilst planning your long-term back link strategy. Ask me about these if you’re willing to do a lot of hard work.
The following is the same example above, but is based off a back link from a .edu and a .gov
.gov/.edu Site 1 with a PR5 has 50 links (the max you want on 1 page) = bleeds .4352 PR
.gov/.edu Site 2 with a PR5 has 10 links = bleeds .88721 PR
.gov/.edu Site 3 with a PR5 has 2 links = bleeds 1.176 PR
So this means that it is important to get back links from high PR sites, as well as sites that have related content.