I just installed Visual Studio 2012, and was pleasantly surprised to find a new storyboard tool based on PowerPoint!
When you run the tool, you get regular PowerPoint with a set of shapes preconfigured to work together that represent common user interface elements:
Here is an example of the SharePoint background shape:
Here is the same background with some common widgets added on to the page:
The new Windows 8 UI elements are also represented as well such as the new Charms concept in Windows 8.
Because this is just regular PowerPoint, you can use these shapes to build presentations, walk-throughs, animations, clickable demos, etc. in the same as any other PowerPoint presentation!