After upgrading my laptop to KDE 4.8 I was unable to start Kwin GLES as default window manager. The only way was to switch it in console:
kwin_gles --replace &
It's not very comfortable. I found this hint on ArchWiki, and adapt it for Kubuntu:
First, you need kwin_gles installed:
sudo apt-get install kde-window-manager-gles
When it is installed, create new file ...
sudo nano /usr/share/kde4/apps/ksmserver/windowmanagers/kwin_gles.desktop
... with this content:
Now you can change the window manager in the system settings: Open System Settings, go to Window Manager, select Use a different window manager: and select KWin GLES.