Initially this came out as a discussion on #ubuntu-cloud (or was it #juju?) when swift CDN backed mirrors were announced for Amazon EC2 cloud.
It is in a way related to servercloud-q-apt-improvements the hash based apt repositories.
Debian has http://snapshot.debian.org/ repository. In essence it has every package, every version, every architecture, ever published in debian (more or less). Furthermore it also makes all of these packages available with standard apt-get, by storing and providing Releases/Packages files made by each publisher run (more or less).
Currently in Ubuntu/Launchpad we do not have such facility.
It is possible to retrieve individual source&binary packages from launchpadlibrarian, but the whole Ubuntu Archive is not available as of that point in time.
Ideally if the Ubuntu Mirror is backed by immutable CDN, it should be relatively cheap to also store and server the repository files over the CDN to create equivalent service like snapshot.debian.org but only for the ubuntu archive.
Another use case for this type of mirror would be the archive that currently holds automatically generated dbgsym packages. Currently dbgsym packages are not available for every single version of packages in the archive. And for launchpad/whoopsie/daisy retraces it has been requested to keep all versions of dbgsym packages and not remove them, otherwise retraces fail and we are loosing important information from submitted core dumps.