KDE on OpenSolaris is like Projects/KDE on Solaris but with some extra setup steps. There are IPS packages available intermittently, when the KDE IPS package server is up (it's a VM at the end of a DSL line - see the archives of email@example.com to find it). Using OSOL as a build platform is possible, but you'll need at least one Nevada machine as well.
The current KDE4 IPS package server is the machine pkg in the domain bionicmutton.org; the IPS server runs on port 10000. This is a fairly standard IPS setup. The bionicmutton domain is Adriaan's and has been previously used to serve up SysV packages as well. The IPS server is in a VirtualBox at the end of a DSL line, so it's not necessarily up or fast. Eventually we will be moving to a more convention IPS repo like pending/ or contrib/.
First you need to set up a pkg authority to be able to get packages from bionicmutton at all. The first line creates the authority; the second fetches a catalog from it and the third checks that at least one of the packages can be found. Only the first is strictly necessary.
pfexec pkg set-authority \
-O http://<host.domain>:10000/ bionicmutton
pfexec pkg refresh bionicmutton pkg search -r KDEgdm-integration
Remember that KDE includes setuid code. Remember that installing packages from untrusted and unsigned third parties is insecure. Remember that the KDE codebase is huge and not extensively tested on OpenSolaris yet. Consider whether you really want to install KDE4 on the machine you're working on. Then decide to do it anyway. You will need KDEbase-apps for things like Konqueror and Konsole, and KDEgdm-integration to be able to choose KDE as a session; other KDE packages may be installed as you need them (such as KDEpim, KDEgames, etc.). There is a KDEconsolidation package as well that pulls in everything we know of.
pfexec pkg install KDEbase-apps \
After installing KDEgdm-integration, you should be able to log out and choose KDE as a session type from the login manager. Then you get a full KDE 4.1.3 desktop. On my machine with Radeon graphics it is very slow to start up and launch applications, but fairly fast after that. There is a discussion on performance tweaking on firstname.lastname@example.org.