I just ran across an article, which describes the use of user interface prototypes as software specification method.

"The user interface specification provides an illustrative requirements specification for a system, because the specification visualizes the functionality of the system in a form that both the client and the supplier understand."

Read the full article at http://www.interacta.fi/ui_spec.html

In her blog on bridging the gap between business and IT, Laura Brandau recently also discussed the form of UI specifications.

"Good UI specifications take into account the data and context of the user within the application. This sort of spec does not replace UI design, but it does help you lead your team through thinking through the UI design and how users will actually experience information within it."

Read the full article at http://www.bridging-the-gap.com/how-to-create-a-user-interface-specification/