Version: (using KDE KDE 3.5.6) Installed from: Ubuntu Packages sometimes my KDE just freezes and it doesn't respond either to keyboard (ctrl+alt+backspace) or mouse buttons. i can just move my mouse and i also see that computer is alive because i see progress in kcpuload. cpu seems to be generally idle. the only way to "unlock" such state is to hw-reset the computer :-(.
important note: this freeze occurres mostly after extensive window switching and opening. also this happens only maybe once per week.
sounds a bit like a X.org bug, but maybe lubos knows something.
If not even ctrl+alt+backspace works then I suggest reporting to xorg developers.