Do all clouds have a multi-tenancy architecture?

“Single tenancy goes hand in hand with those proprietary mainframes that Larry [Ellison] wants his customers to buy because that’s the best thing for Oracle.” — Marc Benioff

Today’s WhatIs.com Word of the Day is single-tenancy. Larry Ellison (Oracle) and Marc Benioff (Salesforce) have been debating whether or not  you can lure customers with promises of a single-tenancy architecture and still call it “cloud.”  Ellison thinks you can.  Benioff says that clouds, by definition, are multi-tenant.