on append() do something

You can override the default append method and cause it to trigger a custom append event. Then bind a handler to an element for that event: http://jsfiddle.net/H8ygx/

(function($) {
    var origAppend = $.fn.append;

    $.fn.append = function () {
        return origAppend.apply(this, arguments).trigger("append");
    };
})(jQuery);

$("div").bind("append", function() { alert('Hello, world!'); });

$("div").append("<span>");