For our Drupal hosting provisioning system, we chose to go for Disk Hosting. This is an open source package, based around LDAP, which is very powerful, but above all the industry standard for storing clients information. The idea, is to make Disk hosting ready for drupal vhosting, so that it can become a standard in drupal-hosting. Our server manager, is also lead developer of Disk Hosting, which was another major reason for using this project.
You can follow the progress of this hosting project in the webschuur.com 'Drupal Hosting' category.