FYI, I built a new machine using an i7 4770 and a mobo with vga, hdmi, displayport, and dvi. I just received my third monitor today, a Dell 2412M, and plugged it in via displayport. The other two monitors are 23" 1080p monitors.
I'm running Mint 15 x64 with Cinnamon, and version 3.9x of the kernel. It "just worked". It picked up the monitor immediately, and now I have a desktop that spans all three displays. I was even able to rotate the display on the Dell to run in Portrait.
I then popped in a live cd of Mint 15 x64 KDE, and it worked just as well. From what I recall, that ships with kernel 3.8.
So, with Haswell, as long as you are running one display on Displayport, the newer kernels will work with three displays. I didn't try any of the effects in KDE (wobbly windows, etc), but everything functions as you would expect.
I will say that there is a little (almost imperceptible) input lag when typing quickly in a text editor (Eclipse java development, for example) in Cinnamon. Response was a tad crisper (though it could have been my imagination) in KDE.
Anyway, I wanted to share. I have been waiting on one of the review sites to specifically review the three-display functionality of Haswell before I purchased a third monitor. I got tired of waiting. I'm glad that was the case - this third monitor will be a boon to my productivity.