Until recently one of the largest sites using Drupal was the Onion. The new White House sponsored site, Recovery.org, has been built using Drupal. There is an excellent article at techPresident about what this means:
“Second, it shows that the White House isn’t putting much stock in the argument that collaboratively-built software isn’t stable or secure enough for government use. (Though one could make the argument that Recovery.gov isn’t exactly mission critical.)”
I think that this is a giant step for open source software as whole, not just Drupal. Systems such as Drupal, Plone, Joomla and WordPress are all excellent systems with large development bases, and when the White House shows that they trust in these systems, it should open up the door for others to be using them as well.