Http headers say 'Server: White House'

I would guess all that is publicly accessible is static and being served from Squid or Varnish.

Good luck taking advantage of any Drupal security hole.

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.

absolutely. I bet Vegas is taking bets on it