I think we're talking about the same thing :D? The service/business logic/whatever you might want to call it interacts with the storage via an interface - in prodcution that interface is implemented by a component that can talk to a real SQL database, in tests I can just create a wrapper around a hash map and use that.

EDIT: What I meant when I wrote that tests are not coupled to the underlying model/details is that with a mock you have to explicitly specify "when called with this return that". With an in memory database implementation you don't need to do anything, the code will just use the interface methods like "getX" or "saveY".