I discovered yesterday that kde desktop effects now work fine, as long as the firmware-linux package is installed; if firmware-linux is not installed KDE refuses to use compositing and does not say why, but the machine no longer freezes. I don't know which of the many updates since May cured the problem (and it would be hard to find out), but guess it was the switch to kde 4.3 or the recent Xorg upgrade. As far as I am concerned this bug can now be closed.