Use this code syntax.
$("<div></div>").html("YOUR MESSAGE").dialog();
this works but it append a node to the DOM. You can use a class and then or first remove all elements with that class. ex:
function simple_alert(msg)
{
$('div.simple_alert').remove();
$('<div></div>').html(is_valid.msg).dialog({dialogClass:'simple_alert'});
}