apt-zeroconf

Have you seen apt-zeroconf? The idea is each machine on a network advertises itself as a apt repo, and a proxy checks them before going out and downloading any new packages.

The really great thing about this is it would be perfect for laptop users - have it sync to my desktop at home, have it download packages directly at work, and I don't have to change anything.

It definitely would need some attention, but it'd be a good place to start.