Interesting question!
I've never had the chance to use a proper multi-touch display, but a couple of thoughts immediately spring to mind.
A large proportion of the physical input into ArchiCAD is keyboard based - typing distances being the main, but also adding text into various dialog boxes - how would this be dealt with? Is an on-screen keyboard usable enough for this purpose or would it get in the way? I wonder how large the screen would have to be to not obscure important stuff underneath?
Another concern would be the accuracy of a finger tip. In order to correctly select or snap to objects close together, there would have to be even more zooming in and out than we do now. I wonder if a stylus would be better suited rather than a chubby finger? (A general question I've always wanted to know is do your fingertips slowly wear away over time with extended use?
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I don't think the user interface as it stands would work terribly well with touch screens. I suspect it would need a substantial rewrite to work efficiently, possibly with a built-in numerical keyboard on the pet pallet/tracker, and better organised larger icons, but I reckon it could be done! I'd love to see some concepts!
I bet Dwight would go for it. Finally he'd be able to wave his arms around and actually produce work directly!