Most of what is quoted in this smashingmagazine article does a good job at demystifying the operation of the Google engine. Pay particular attention to the parts about the fact that PageRank (PR) is ONLY ONE of many factors used in determining relevance.
Many of the folks we deal with are under the assumption that having a high PR means they will also have better SERPs.
The truth is that PR is only one of many variables that go into creating a good result set for a specific query from Google. More specifically there are two areas that deserve more focus and understanding:
Google HIGHLY considers the diversity of results on the page (more topics covered means you might have one close to what you were looking for). Also;
Google interprets the context of your page (aka your page content, and the pages linking to you use anchor text to describe you) and the context of the individual searching (using their search history) to determine and boost a pages position in the rankings.
There are other known streams that contribute to your SERP rank but these two have the broadest impact that we know of.
PR is important but it's like having a nice car in a race without any gas. You might have a nice PR but you aren't relevant so you aren't going to win the race. Lower PR sites that are more relevant WILL zoom right past you. You can test this yourself on google today:)