Setting window behaviour to anything else than "click to focus" in KDE 3.5.5 (FC5 and FC6) results in lots of window flicker, lots of CPU load and an unusable desktop. Upstream bug and a potential (untested by me) fix at http://bugs.kde.org/show_bug.cgi?id=135250
it's already available in FC6-update testing tree. *** This bug has been marked as a duplicate of 213219 ***