In a sense, Google is the largest crowdsourced project of all-time. It's a lot like Reddit or HackerNews in that every link is an upvote, but the genius of Google is that each link carries a different weight, and that links are a natural byproduct of using the Internet. In short, the people contributing to the crowdsourced ranking system don't even realize they're doing so most of the time. They're just doing what they like and leaving a byproduct of doing so (links, social signals) that Google can use to tell you which sites people consider valuable.
But that means that once people realize what Google is using to rank, they can mimic those signals, and sway the algorithm in their favor. The problem Google is going to run into is that once spammers can closely (and at times programmatically) mimic what is happening "organically," Google's algorithms cannot tell the difference.
Right now Google's approach seems to be ignoring or highly devaluing portions of the Internet that have been overrun with spam. Article submissions, blog comments, and now apparently guest posting, which sucks for people that do really high quality, organic guest posting; for Google that has to be collateral damage. Spammers will simply move on to the next portion of the Internet, mimicking what Google still uses as a ranking signal. It's an endless battle.
One of the big things I see happening now is entire website hijackings (I've been meaning to email you about that, Matt). I did a quick little report on the search engine results for "Viagra," and 81 of the top 100 are hijacked websites, including a client I have to upgrade to a newer version of Drupal, as the older one has been compromised. I don't know if or how Google will win this battle, but it's far from over. I honestly feel like the new way we gather data to rank websites, and what will be successful in 25 years, will have to be completely unrelated from what Google is doing now, and much harder to manipulate than spreading links all over the Internet.