Mirroring PyPI becomes a recurrent need for Zope development, because zc.buildoutmakes a lot of package downloads to build one application.
This is useful when you are working in an intranet with limited web access or when you want to speed up download times. It also makes things safer: if PyPI is down and if developers computers don't have caches, having a mirror will save your day.
While PyPI has proven its robustness (it is 100% up for months now as far as I can see), having mirrors makes a lot of sense.
We have created a small mirror application here at Ingeniweb, that we use for our buildouts needs. This work was thaught and created by my colleague Bertrand Mathieu.
It is a smart proxy that will download packages at PyPI everytime they have been asked by a buildout or an easy_install client. When the package is downloaded, it is kept in the proxy side for any new requests. This means that after a while, the proxy has its own collection of packages that corresponds to the real needs and will not query PyPI anymore.
This approach avoids having to download and synchronize PyPI with crons, which is a heavy process since PyPI weight several gigas. The caveat of course, is that it won't be able to get a new package if PyPI is down.
Take a look ! http://pypi.python.org/pypi/iw.eggproxy
By the way there is an interesting sprint coming up on all these topics, in Germany :