That's a fairly complex system you're describing, not something we're prepared to integrate into zenphoto at this time due to the philosophy of the project. The multi-user aspect sbillard was talking about was a simple but useful system of multiple manually-entered users with different rights.
We want to keep Zenphoto as simple as possible, and its primary purpose is to be a simple gallery solution for a person or small group with their own web site. Registration, user management, user banning or disapproving, photo approval, etc. are all features of complex multi-user galleries (or even social photo sites), and would introduce more security issues, debugging issues, social issues, user rights issues, etc. etc.
If you're looking for something that requires all those features, then perhaps a larger gallery system is better suited for your needs, such as Gallery2 or Coppermine.
We can also add that feature to the feature poll, and if enough people express interest then we can consider it more formally.