Multihead, where there is more than one physical screen and one X server per physical screen (not to be confused with xinerama, xrandr, mergefb, etc.), and Plasma Desktop is getting into a rather usable state thanks to testing and feedback from users with those systems that goes beyond "it doesn't work". Thanks to the digging and debugging work of several individuals, my "coding in the blind" has produced finally produced useful results as of the 4.5.3 release. There are still some KWin issues, apparently, but plasma-desktop is pretty well there.
Today I added a boolean multihead variable to the Plasma Desktop Scripting, and yes it is documented. This allows scripts, should they need to, do different things in a multihead setup.
There are some unique annoyances with multihead, though. For instance, when enumerating the screens with Kephal it will report that there are N screens, but each the origin point of the geometry of each screen will be (0,0) meaning that it looks like they overlap, even thought they don't. This leads to some awkward behavior both in the initial layout set up as well as things like repositioning panels on screens other than the first one. Nothing fatal, but not optimal either.
An approach occurred to me today that might make this easier to handle: make plasma-desktop's Corona "lie" when in multihead mode and say it has just one screen and map screen "0" to the real screen id transparently behind everything's back. It's a bit of a hack, but so is multihead. ;)
I have no idea how well this works due to not having a system to test it with, though in theory it should be an improvement. What I need are people with multihead systems who are running trunk to try this patch to plasma-desktop and report back what happens. I expect that it does nothing for the panel moving issue, but other things should improve or at least not degrade. If that is the case, then fixing the panel moving issue becomes trivial by extending this approach there as well.
Please test if you can and let me know on plasma-devel at kde.org or in a comment on my blog here. Cheers.