Regional & Accessibility->Keyboard Layout ->Xkb Option->Layout Switching->[x] Alt+Shift change layout.
The generated "Command" is: setxkbmap -option grp:alt_shift_toggleIt works fine in KDE 4.1.3. It even generates the same "Command". Furthermore, loading the KDE 4 keyboard layout applet in KDE 3 works and I can switch with Alt+Shift. The KDE 3 one doesn't. What am I missing?