Distributed computing on a business venture like a search engine would be a nightmare within a nightmare. Google can barely keep up with search engine scammers right now (some would argue they aren't keeping up) and those scammers are working on a trial and error / poke and prod basis to find out what they can do to sway results one way or the other. Imagine if a central compiler not only had to create algorithms similar to what Google uses now, but also had to include in those algorithms a flawless method for verifying that data coming in from distributed centers had not been tampered with or entirely falsified. In short, I can't see it happening anytime soon. Seti@home distributed computing works because there isn't any financial gain directly tied in to tampering with Seti data. In other words, there's no motivator for large numbers of people to screw with the system. On the other hand, search engine results provide immediate and immense profits to anyone who can control (or even sway) them. Heck, I know some SEO professionals that make in excess of $100 per HOUR to sway search engine results in favor of a client.