http://buytaert.net/whitehouse-gov-using-drupal
Big, exciting news! The flag ship website of the U.S. government, Whitehouse.gov, just relaunched on Drupal. This is a big day for Drupal, and for Open Source in government, and something all of us in the community should be very proud of.
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Being one of the world's largest consumers of computer software, the U.S. government is not new to Drupal. Several agencies, including the Department of Defense, the Department of Commerce, the Department of Education, and the General Service Administration have been using Drupal, for example. Drupal adoption is growing rapidly within the U.S. government. However, Whitehouse.gov switching to Drupal goes above and beyond any other Drupal installation within the U.S. government, and is a fantastic testament for Drupal and Open Source. It will raise awareness about Drupal across the U.S. government, and across all governments world-wide.
Personally, I'm thrilled by the idea that Drupal can help governments provide greater transparency, higher velocity, and more flexibility.

Disclosure: my company Acquia was involved in the development of Whitehouse.gov in partnership with General Dynamics Information Technology, Phase2 Technology, Akamai, and Terremark Federal Group. Additional details can be found in this TechPresident post (PDF version).