For traffic that consists of many small, relatively rare files (which is most HTTP traffic) you would have to do some proactive cahing anyway. I want JQuery 1.2.3. I ask your computer. Your computer doesn't have it, either because it's a rarely used version, or because you cleared your cache. Instead of returning some error code, your computer asks for it from another node, then caches the file, then returns it to my computer. This kind of stuff will be necessary to ensure high availability, and incidentally it would make it hard to see who is the original requestor. It should actually improve privacy. Also, this scheme could be used to easily detect cheating nodes that don't want to return any content. (They usually wouldn't have an excuse of not having the content you requested, so if a node consistently refuses to share, it can be eventually blacklisted.)
> I want JQuery 1.2.3. I ask your computer [...] returns it to my computer
Good ! Now I know you have JQuery 1.2.3, which is vulnerable to exploit XYZ, which I can now use to target you. This is one reason why apt-p2p and things like that can't be deployed in large; it's way too easy to know what version of what packages are installed on your machine.
