Still, it strikes. That is some OS thing.
The desktop manager always reacts to key strokes, I guess it is located at a very close level to the keyboard driver. Also the task switching via APPLE+TAB works. It seems that this state is reached when I switch to an editor task, such as WingIDE. I can not really determine when it happens, because it doesn't happen all the time.
But I know how to quickly get the keyboard working again! I simply have a TextEdit opened all the time, I task switch there and it works again. Strange but true ...