I've worked on a Drupal project with about 1 million nodes. We added transactional support and it wasn't too hard. You'll need to patch the core of course but this shouldn't be a major concern for an enterprise application with good support and documentation. I was working as the observing pair programmer on the transactional support. I think it took us about a day.
Edit:
I've been working as a Drupal Developer for a few years now. And recently, I have revised my position on Drupal in relation to best practices and enterprise application.
I don't think Drupal is particularly suited to the Enterprise space because:
- Drupal's testing framework is too cumbersome (a domain specific language would be great)
- There are too many contributed modules of poor quality
- Drupal's content model resides partly in code and partly in the database schema
- Drupal's developer community have become focused on sexy software rather than quality software
- Drupal doesn't have mature developer tools (drush is changing that) or a development environment built in.
- Drupal Development is browser and UI centric.
- The centralized nature of Drupal.org/projects, CVS and Drupal's policy on Contrib Projects inhibits the evolution of individual software projects
Also: The enterprise Drupal Application I was once working on has now been ported into Rails.
