Answer One: Yes
- internet_search://"drupal in the enterprise" <- use this exact phrase
- Drupal "Success Stories"
- Student Activities Supports 170 Drupal 6 Sites at Texas A&M
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.
- Drupal 7 Database Plans
- Drupal 7 Database Update
Answer Three:
- Open Source Content Management Systems
Answer Four: (Update: 2010-02-03 11:25:04)
- see also: https://stackoverflow.com/questions/1715811
