During the Meet The Crawlers session at SES, someone asked why all of their pages weren't indexed. Google's answer: Every search engine crawls in a different way. Mentions vs indexed. There are instances where we know about the url, but we did not crawl it. Your site may not have enough PageRank for us to do a deep crawl.
This isn't any earth-shattering news, but it does underscore one of the reasons why PageRank still matters.