The new 28" Dell only accepts input at 30Hz.

Some of us were holding out hope of it supporting 60Hz via MST over DisplayPort, however it's now confirmed that 30Hz is it.
As for the 24" Dell UHD monitor -- UP2414Q -- it does not currently work with the nMP at 60Hz. I have tried it and subsequently returned the display. I now wish I would have kept it for my HP workstation and will re-order when a good coupon appears. Presumably this display will eventually work with the nMP, we just don't know for sure when or how the compatibility will be fixed. It could come via a firmware update for the display or it could come with improved drivers from Apple/AMD. In fact, the latest 10.9.2 developer seed released a few days back has more video driver updates in it, specifically for the nMP, and I have not tried with that release.
With OSX 10.9.1, the display worked properly at 30Hz and 24" is a good size for running HiDPI mode for a 1080p equivalent desktop with "retina" support. However, for 60Hz operation, the monitor did not work. We had a mess of garbled colors and couldn't make out much of anything. With the 10.9.2 beta seed, the monitor worked properly on the primary stream side (right) and the left hand side of the display showed sparkles, flickers and drop-outs. It seemed to be a timing issue. There has subsequently been a new 10.9.2 beta release, which I mentioned, but I have not heard if anything was fixed for this display and I have already returned the one I originally purchased.
I regret the return now, as I'm sure this will all get sorted and I do have other ways to use the display if not. Unfortunately, I let the 28" Dell influence my return decision and I was unaware of the 30Hz limitation on that display at the time. So I feel like a dope at the moment. Ha!
There are reports floating about that the 32" Dell UP3214Q also has the same incompatibility issues with the nMP, I have not seen or tried this display. I have however worked with the ASUS PQ321Q display and it works properly at both 30 and 60 Hz. As does the Sharp PK-321, which Apple is selling. The ASUS is simply a re-badged Sharp anyway. Personally, I've found the 32" size to be awkward for UHD 3840x2160 use. It's too big of a monitor to run in HiDPI mode for a 1080p workspace and there is inadequate support in current OS's (Windows or OSX) to make use of the display at native resolution.
The 32" 4K monitors are nice however as a secondary display where we can work on photo retouching or as a playback/view monitor for UHD video editing. I think the "killer app" for Hi DPI displays will arrive later when we have better support for HDMI 2.0 and the upcoming DisplayPort 1.3. I would love a 5120x3200 30-inch or a 5120x2880 27-inch "retina" display. That would give us a nice balance of screen real-estate with sharp text and other display elements and the ability to use 1:1 pixels in apps that can benefit -- photo / video work, CAD, etc..