Short answer: I have this exact configuration, and yes, the newest rMBP 15" w/Nvidia card will drive the UP3214Q over HDMI at 4k and 30Hz. It will drive it as a separate desktop with the laptop open, and it will drive it as the sole display -- either way works fine.
Longer answer: You should be aware of 2 things (that I wish I had been more aware of before purchasing the monitor)
1) If you are used to 60Hz or higher, the 30Hz limitation over HDMI is noticable, even in day-to-day productivity work. The mouse lag is very noticable and annoying to me.
2) Scaling/HiDPI support [the way the integrated retina display works] is virtually nonexistant at the resolution you'd want to run it at. What this means is that you either run the monitor at a lower resolution (and everything is blurry), or you run it at full resolution and everything (menu, dock, fonts, etc) is extremely small. You can use SwitchResX to enable scaling support at odd resolutions (for instance, I've successfully gotten HiDPI/scaling to run at 1920x1200 and 2048x1080, with black bars at the edges), but it's not an ideal resolution for this size monitor. I want to match the TB display resolution (2560x1440) in HiDPI mode, but I've been unable to get it working after hacking away at it for 2 days.
I'm hoping that Apple will fix these limitations. Multiple people have reported that MST over DP1.2 works on the same hardware if you boot into Windows, so the hardware support is there -- it just needs to be enabled in the drivers by Apple. Whether they actually will or not is anyone's guess. In the meantime I (and many others) are in limbo wondering if we should return these 4k displays, or keep them and hope "proper" support comes. Since it's not a specifically advertised capability, Apple isn't under any obligation, but I think there's more than a few people hoping that Apple does right by the users and enables the full functionality that the hardware supports.
