Getting Real: "Functional specifications documents lead to an illusion of agreement. A bunch of people agreeing on paragraphs of text is not real agreement. Everyone is reading the same thing, but they’re often thinking something different. "
I'm not sure what the answer is, here. Certainly UIs need to get more prominence in the development process (see Palm, Apple, Sony for reasons why)... but this needs to be done within realistic parameters - there's no point in designing a user interface which can't be implemented.
I also believe that, especially with mobile, it's difficult to design an interface without playing with it - without actually having it on a target handset so that you can feel what's right and what's wrong about it. Such things can be difficult to articulate or document.