Mac OS X never took off in the same way that Windows did. But I suspect that Google Chromebooks will either push users away from Mac OS X towards iOS devices, or have a stranger effect: bring iOS users of the iPad and iPhone closer to its Mac OS X sibling.
With three major desktop operating systems — Windows, Mac OS X and the million variants of Linux — there will soon be another, in a manner of speaking. Google Chrome OS will be pre-installed and supported by Chromebooks — dedicated light hardware which will run the cloud-based feature set of the operating system.
But while the vast gap in market share between Windows and Mac OS X has slowly narrowed over the last three or four years, mobile operating systems have had their time to shine. But as the majority now access web services and sites over other applications pre-installed on local machines, Apple could have most to lose in the desktop operating system space, by reducing its already little by comparison user base to lesser-capable devices like the Chromebook.