OpenIndiana, the spork of OpenSolaris just released the first iso images, so I grabbed one and gave it a go inside VirtualBox.
It booted just fine so I added the KDE4 publisher and tried to pkg install KDEgdm-integration - the basic KDE desktop...
... and it went just fine. There were many dependencies that were downloaded from opensolaris.org during the process. I haven't really investigated whether it's because the OI team hasn't sporked them yet or if there was some other problem.
After a logout I was able to login to a KDE 4.5.1 session and it did not feel much different from one in OpenSolaris.