KWin will come with its own compositing and effects, so there will really be little need to make KDE work with Beryl. The burden of making Beryl work with KDE lies on the Beryl end, though.
The advantages of KWin having its own compositing is that the effects willb e built on top of an already stable and existing window manager, not the other way around. It also means that we don't lose any of the KWin features and peculiarities.
So yes, we will probably see spinning cubes soon...