For the growing universe of developers turning to Drupal as a solution for mission-critical or highly ambitious applications, the question is less and less "can we build it?" and more and more "how do I scale it?"
As we all know, Amazon's EC2 service is really beginning to mature as a web platform. Acquia is using it for their managed hosting, and many other companies have proven that with the right architecture and setup, "the Cloud" is more than just a buzzword.