Well, you're right that it requires centralization, but at least the amount of data you need to retrieve centrally is that much smaller. It would benefit the global infrastructure greatly if I could retrieve websites over LAN if they'd been cached by a computer there.
My wife and I visit a lot of the same sites, even see a lot of the same dynamic content (same facebook friends, etc). Surely there's some community effect that would make this a huge bandwidth win.
Careful: this gets you dangerously close to privacy violations. You may not want certain people to know that you are fetching a piece of content. "Hmm, the profile picture for this specific friend loaded suspiciously quickly.."
A similar mistake is security updates over bittorrent: a real-time list of vulnerable hosts.
My wife and I visit a lot of the same sites, even see a lot of the same dynamic content (same facebook friends, etc). Surely there's some community effect that would make this a huge bandwidth win.