I think they'll call it "Mac OS X version 10.9.9 Extreme". The "Extreme" will be an integral part of the name, of course.
But back on topic:
Mac OS X (in version 10.4) already incorporates a scalable UI. If you install the developer tools, you can actually use Quartz Debug.app to scale the UI. (Only apps started _after_ the setting will inherit the change.)
What we _see_ there is that there are quite a bunch of bitmap graphics in the OS' interface. Scaled, they start to look ugly. I think a switch to 300ppi displays would be the wrong step.
The first step should be to use vector graphics in the UI. And _only_ vector graphics. Maybe some high-res textures somewhere. But better not. Because actually, I think we should have 150 and 180 and 270 and 300 and 600 and 765 ppi screens. The OS should be completely scalable - not set to an either/or setting of 72 or 300 ppi.
And about the internet graphics: I guess browsers would have to simply scale 72 dpi images (looks ugly, but at least they get the size right, then, and the page layout) - and more modern webpages would adopt a way to deliver images based on user settings (i.e. "I want, say, 'middle resolution' images for my high-res screen...").
We've been talking about 300 ppi screens like forever (or since the Macintosh II or something) - and they simply haven't happened. Just like we haven't replaced our harddrives with much faster RAM disks (or flash-RAM disks), although _that_ had been proposed at various times. Harddrives simply got bigger/faster/cheaper - so the move to non-moveable parts didn't make real sense. Or like those incredible e-paper solutions. You know: Displays for PDAs that only need power for changing the black-value (or white-value) of a pixel. Rumours of such displays were running wild back when the Newton was old and the Palm was new.
The 300 ppi display remains a myth - at least for now and probably at least another five years.