How can I setup Git/Drush dl/make caching of Drupal projects? I'd like to setup the ability to have Drupal projects cached locally when I do a Git clone or Drush dl/make, so that if I happen to be offline or just to save bandwidth, when I Git clone/Drush dl/make a project that has previously been downloaded it will pull from the cache instead of from D.o., possibly after pinging D.o to see if there is a newer version (if online).

I'm sure I saw something about this once before, but haven't been able to track down the specifics.
There is a blog post about locally cached git repositories here: http://randyfay.com/node/93