There are plenty of Google hits on that topic, but so far I haven’t found one that gives the right answer. Here it is:
% dconf write /org/gnome/desktop/wm/preferences/focus-mode \
"string 'mouse'"
To switch back:
% dconf write /org/gnome/desktop/wm/preferences/focus-mode \
"string 'click'"
This works in Gnome 3.4, and I assume (but haven’t verified) that it also works in Gnome 3.6. However, given the recent developments in Gnome land, I will probably move away from Gnome altogether.