I'm pretty sure the security issue (Everybody knows you have old packages) is a good point against this system, but it's true, I'd love to see this system more widespread.

For the local network part at least, it's really not that complicated to implement, all you have to do is to listen for announces on the network and ask those peers before asking remotely; there is a standard example for archlinux in pacserve (http://xyne.archlinux.ca/projects/pacserve/) with my own very crude reimplemantation (https://github.com/rakoo/paclan)