Most likely Varnish, which is made possible by the Pressflow Drupal distro, which among other things, cleans up the http header problems in Drupal 6: http://fourkitchens.com/pressflow-makes-drupal-scale

Much of Pressflow's codebase was backported from Drupal 7, so its advantages will be available to all Drupal sites after the stable release of version 7 sometime early next year.

The change to Drupal 7 that make better http headers (now in Pressflow 5 & 6) will allow, afaik, for any reverse proxy, but varnish is definitely one of the easier to use.

All Drupal 5 and 6 sites can use these features by substituting the Pressflow core.

Now I'm wondering how the US government will contribute back to the Drupal project.