Poking around in the new 10.6 APIs, I've become more convinced that it will indeed be Mac OS X.
Without becoming too technical, in 10.6 Apple has significantly expanded the information that NSEvent is capable of giving: views can now respond to things like pinches and swipes, and other multi-touch information.
Currently, these events are generated from the user's actions on a later model trackpad (from the MacBook Air onwards).
It's possible they'll be confined to just that, but it seems like a lot of work to significantly revamp the event model just for a few trackpad gestures that most people don't even know about.
(For a long time apps like Preview have responded to pinch zooming, but most people I talk to are surprised when I point it out to them. Apple hardcoded these gestures in, but now they're in AppKit for all developers to make use of quite easily and, significantly, they are not quite the same as UIKit which the iPhone uses.)
What if the new events are intended for a larger use: instead of a trackpad, a screen...