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?
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)