[wishlist] Implement delta updates.

System updates are important for several reasons that everybody knows (security fixes, bug fixes, important new features, etc).
However the amount of updates to download after a fresh install is really high. For example as of now Ubuntu Gutsy asks for roughly 250 MiB of updates; so it means that less than 4 month after the release of Gutsy you have to download a 700MiB CD and then update more than one third of it.

Possible solutions (but totally different approaches):
a) make update system "delta updates capable";
b) release a bugfix CD x months after initial release (as happened for 6.06.1, 6.06.2 and always happens for Debian stable).

Let's see pros and cons of a) and b).

a) DELTA UPDATES:
Related (interesting or for comparison) links:
https://bugs.launchpad.net/ubuntu/+bug/137190
https://hosted.fedoraproject.org/presto
http://fedoraproject.org/wiki/Releases/FeaturePresto

pros:
- less mirrors bandwidth usages (and several possible side effects such as number of mirrors increase);
- possibility to make updates also for people with slow internet connections or with connections which cost is based on duration time or transferred data.
- makes update process faster.
cons:
- needs work on packaging system;
- installing an updated package could be problematic in some cases. For example assume that the package you want has already been updated but you haven't already installed it. Possible solutions are: download and install both the original package and the delta, or download and install the updated package. The first solution is not really great because you have to download more things than the present system, the second option requires that mirrors keep both delta and updated package.

b) UPDATE ISOS:

pros:
- less updates required after fresh install;
- can ship installer / kernel bugfixes;
cons:
- requires mirror resync and torrent reseed.

Proposed solution: ship only one updated ISO at half the time between a release and the following (e.g.: 2 or 3 month after the initial release).