muiti site

ok so we have plenty of people who are able to do a multi site install of Drupal where they are the host and the developer, or don't have others working on the same site.  I have a situation where I have multipul users that should not have access to each others files and I don't want to have to create additional ftp accounts.

I am on a LAMP server and want to be able to offer my customers the option to have a 'one install to rule them all' set up, minimizing the time it takes to upgrade the system.  Drupal 5's overrides system works great for this.