I ran into the perfect situation this week that defines why you should not put UI assumptions in Use Case (or task) documentation. A customer decided that they really wanted to change a UI from the proposed tabular approach (with potential popup windows) to a tree view. If the use case mentions things like "double-click", "press OK", and UI phrases of that nature, the use cases would need to be re-written. Without those UI assumptions they should be fine.
So stay away from UI assumptions in use case documentation. Using a model/view/controller design, the view is just the view. Some views are cheaper than others, but using MVC the view is just the view is just the view.