Based on this[1] discussion, it seems like Thunderbolt 2 equipped Macs have DP1.2 and should be able to drive it at 60hz. It seems like someone with an Asus 4K screen had trouble with this in OS X (Win 8 worked).

TB1/Display Port equipped Macs will be limited to 30hz.

[1]https://discussions.apple.com/thread/5475430?tstart=0

The most recent retina macbook pros have serious driver problems with 4k monitors. Yes, after the December update too.

Here's my Amazon review of a Seiki 4k display: http://www.amazon.com/review/R4OJ5D5RPILCH

Only the latest MacBooks with Haswell chips support 4k over Thunderbolt 2

Do you know if they will they run the screen in a scaled mode? (i.e., 1920x1080 logical pixels, while using retina scaled assets)

Yes they should be able to. OS X can already do that on regular screens.

This monitor doesn't support scaling. If you push 1080p, it will not fill the screen.

That's not what HiDPI mode does; rather than rely on the (uniformly crappy) on-board scaling logic on the display, the computer sends a full resolution picture that it has scaled already. And there's no reason to think that this won't be available with this display.

I want to believe!

Late 2013 Haswell Retina Macbook pro could support 2 upto 3 4K external Display I think. If Win 8 worked 4k with 60HZ,just wait for a new OSX 10.9 update.It should bring 4K with 60HZ support. If using HDMI cable,only support 4K at 30HZ.

As an additional question: Are there any Macs with support for two such monitors?

The Mac Pro.

Thanks!