I understand the general idea about unit testing and have used it in scenarios where complex interactions were happening in a system, but I still have a question about all of those principles coming together.

We are cautioned about not testing the framework, or the database. Good UI design does not lend itself to non-human testing. UI interaction in general is excluded in MVC frameworks. In many apps what is left?. 37signals talks about extensive unit testing, but in an app like Basecamp or Backpack, what exactly are the types of things being tested through appropriate unit tests? What would 100% code coverage mean in that scenario?