A WebmasterWorld thread asks "How does Google determine which pages to crawl?" Google didn't always crawl and index pages as they do now. With the Big Daddy update Google adapted their crawl priorities, which was around April 2006.
Google now bases the crawl priorities based on several factors, one of those factors includes PageRank. As far as I understand it, pages with higher PageRank will be crawled and indexed quicker than pages with lower PageRank, as a general rule.
That is one of the reasons people recommend placing links to your most important pages on your highest PageRank pages (i.e. homepage). One it will increase the PageRank of those pages and it will also give the bot easier access (higher level access) to the page.
Back in the older days, it was easier to get Google to index and rank all your pages on your huge dynamic site, if the pages were search engine friendly. Now even indexing requires page popularity and trust factors. Don't even get the SEO community started about being indexed but being in the supplemental index. :)
Forum discussion at WebmasterWorld.