Earlier quoted context omitted.
I have also thought that distributed crawling with the help of browser extensions, and/or clients like folding@home, could be a good idea. But how to deal with "spam injections"?
After a certain scale I think you can let clients do double-work and let the most common crawl data, among different clients, win. And since you control what URLs need to be crawled, you protect yourself against rogue clients sending arbitrary URLs. There certainly are a lot of elegant ways to reduce spam for this particular problem imo.
I'm not worried about the URLs, but the content of the URLs sent back.
Say the server tells a client to crawl a CNN article. The "hacked" client sends a fake CNN article back.