knockout + jquery chosen binding handler http://jsfiddle.net/tomazy/6A57J/

knockout-chosen.js

ko.bindingHandlers.chosen = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext){
        var $element = $(element);
        var options = ko.unwrap(valueAccessor());
        
        if (typeof options === 'object')
            $element.chosen(options);
        else
            $element.chosen();
                
        ['options', 'selectedOptions', 'value'].forEach(function(propName){
            if (allBindings.has(propName)){
                var prop = allBindings.get(propName);
                if (ko.isObservable(prop)){
                    prop.subscribe(function(){
                        $element.trigger('chosen:updated');
                    });
                }
            }
        });        
    }
}

For some reason I had to add some setTimeout to the $element.trigger('chosen:updated') to get this gist working.