Version:           Unbekannt (using 4.2.00 (KDE 4.2.0), Kubuntu packages)
Compiler:          cc 
OS:                Linux

I am unable to offer exact steps to reproduce.
Description:

The keyboard input suddenly stops working in all KDE applications (thus making the whole session unusable). There are no error messages, neither in /var/log/messages nore in .xsession-errors or kdm-log or anywhere else I could look with only the mouse working. Interestingly the virtual keyboard does not work, too. This also explains that if I connect a USB keyboard to my notebook it shows up correctly in /var/log/messages but it doesn't change the situation in the current kde session. I first though it may be a hardware error, but when I logged off from KDE, I was able to switch to another terminal with ctrl-alt-f? . 

I also noticed that this happens when I quickly change windows mit the alt-tab combination. I have had this problem about 5 times since now.

Sorry that I cannot deliver a better reproduable report. If I would know what of QT or KDE is responsible for handling keyboard input I would try to hunt the bug down...