Open Source GUI was supposed to be available in the middle of December, but the KDE Release Team has decided to release KDE 4.0 it in January.
The release has a couple of essential issues which need to be fixed before it is released, including the crucial visual appearance. There is also a bug in certain codecs of xine that cut off audio fragments prematurely.
The developers say that the KDE Development Platform and a lot of applications are considered stable and quite usable. The developers are confident to be able to release a more polished and better working KDE 4.0 desktop in January. The changed plans involve releasing on January 11th, 2008.