[Shameless editing by JeffGrigg...]
My IdealUmlCaseTool would be an electronic whiteboard, a combination of a large LCD display and a cordless digitizer pen. See http://www.wacom.com/productinfo/pl300.html for a small example, a 10-inch 1024x768 LCD display with integrated cordless digitizer pen. That's too small still, but make it say 20-inch and double the resolution and we'd already have something rather useful. Wall-sized would be even better. Imagine a whiteboard where you could save your diagrams, recall and edit them later, and paste them into design documents and PowerPoint presentations. -- DavidPrice
Then imagine a Wiki on that...
Put the digitizer on the table and the view on the LCD or projected on a screen or wall and you have a product already quite good in making useful diagrams. It is called AutoCad. It has been available to do this for over 15 years. And it can be programmed as well. There's no reason someone skilled in programming could not make it behave as a Wiki does. Let's call it CadWiki! -- DonaldNoyes
See also ElectronicWhiteboard and ElectronicPaper.