Isn't the whole point of an interface that multiple classes adhere to a set of rules and implementations?
Strictly speaking, no you don't, YAGNI applies. That said, the time you'll spend creating the interface is minimal, especially if you have a handy code generation tool doing most of the job for you. If you are uncertain on whether you are going to need the interface of or not, I'd say it's better to err on the side of towards supporting the definition of an interface.

Furthermore, using an interface even for a single class will provide you with another mock implementation for unit tests, one that's not on production. Avner Shahar-Kashtan's answer expands on this point.
