https://wiki.debian.org/DebTorrent
This could take off it only a big player like Ubuntu pushed it. I don't see why we depend on a set of centralized servers for a bunch of files that a huge number of people download on a very regular basis.
And yet, the idea has been stagnant for years.
Edit: By which I mean, it works, but not enough people use it.
See this alternative that uses similar idea but not real Bittorrent, they worked around the 1st problem: http://www.camrdale.org/apt-p2p/
I still don't see why Ubuntu/Debian/et al don't take this (or something like it) up in a more official manner. I can see why it's not a default of course, but it could be made a question during installation for example.
An additional benefit would be that you'd be able to source packages from machines on your local network, with fallback to the internet, and it would all be pretty much automatic and configuration-free.
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)
I'm aware that you can swap out the PPAs as needed, but I would really like something distributed and decentralized.