Using Expression Blend’s forthcoming Sketchflow Feature for Requirements Elicitation and Prototyping
There are a number of tools people use for requirements elicitation—Visio, UML tools, PowerPoint, and some wireframe tools. If you look at the article at InfoQ: Using SketchFlow to Create Better Prototypes, you can see how you can lay out a use case flow and screen mockups in Expression Blend 3. Sketchflow is designed particularly for prototyping or storyboarding, and has nice features for feedback, data binding, and navigation.