There is a great case study available on Acquia.com that details the development decisions that were made for the family of Lifetime Network sites, all of which were built in Drupal. If you're interested in issues of Drupal scalability, and how a Drupal site can be built to handle 50 million page views a month, and large spikes in traffic, it is definitely worth a read. myLifetime.com was, at the time of this case study's writing, the 500th most visited website on the Internet. It is very excited to see Drupal handling this level of traffic so seamlessly. As the authors of this study point out, Drupal does not come out of the box with this level of capability, but because Drupal provides a very flexible architecture, developers were able to make the choices that would best suit the Lifetime Digital sites. These choices included multiple databases, extensive caching, and having Drupal 5 and Drupal 6 sites cooperate as necessary. You can read the case study here.