when I upgraded from gnome 3.4 to gnome 3.6 I realized that I could not change my keyboard layout using Alt+Shift anymore. so I installed gnome-tweak-tool and in Typing tab I selected Alt+Shift from Modifiers-only input sources.
now I can change my layouts using Alt+Shift but it is really slow ... what should I do ?
it is because now in gnome 3.6 gnome handles input sources instead of x. you can disable gnome from handling keyboard layouts :
gsettings set org.gnome.settings-daemon.plugins.keyboard active false
then install qxkb and configure it for your own.
now x handles your keyboard layout and it is fast :).