After add the new option, to fire the 'change' event is missing
for prototype I've added the following line finalizing Chosen.prototype.select_create_option:
if (typeof Event.simulate === 'function') this.form_field.simulate("change");
Seems that the jQuery plugin already tiggers the 'focus'.
I don't know how to use it and I don't have the time to do it
so, if it matters for you, the Prototype event is missing there...