Is anyone out there using Drupal for large scale, business critical enterprise applications?
Does Drupal's lack of database transaction support dissuade potential users?
Are there any other lightweight web-frameworks based on dynamic languages that people are using for these types of apps? What about Java portals such as JBossPortal or Jetspeed as an alternative or a Drupal + J2EE hybrid architecture?
Having helped to create drupal sites for Popular Science Magazine and soon another magazine (which I'm not sure if I can name yet?), I think it's quite suitable. We've also done an intranet site for BMW and Pregnancy.org.
It may not be perfect yet, but it's quite suitable.
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:
Also: The enterprise Drupal Application I was once working on has now been ported into Rails.
Answer One: Yes
Answer Two: It depends
There are surely some who have concerns about this issue. Drupal's database support and schema have been subject to some scrutiny and criticism over its evolution. That is likely to diminish if some or all of the planned enhancements make it into Drupal 7. This is the one out of your three questions that cannot be easily and definitively answered by searching the internet.
Answer Three:
Answer Four: (Update: 2010-02-03 11:25:04)
My company is using Drupal to build the public facing website for one of the largest universities in the state that I live in. So far we have been pretty successful with it.
We use concrete for a bunch of other clients as well and I think we're generally happier with concrete although it is not nearly as wide spread used as Drupal.
I've used Drupal to build a combined public-facing site for an organization that also included internal features for employees such as meeting organization and internal document management. Mostly it worked out well and they are happy with it, and very happy not to have to manage two separate sites and / or something like sharepoint.
The one place I found Drupal to be a little lacking is in its LDAP support. You can use it just fine for authentication, but there are some bugs in the modules that try and bring LDAP groups down as roles, if this worked more seamlessly I would be completely on board with the "Drupal is enterprise-ready" sentiment, but it's not there yet.
That said, you can do a lot with content types and taxonomy-based access control to customize what content is available to what roles, something that is a requirement for most intranets.
If you will see the list of sites here http://buytaert.net/tag/drupal-sites and here http://www.drupalsites.net/, you will see that Drupal is being used to build small, medium and large scale sites!