var myselect = $("select#foo");
   myselect[0].selectedIndex = 3;
   myselect.selectmenu("refresh");

$('#PAGE').live('pagecreate',function(event){
    var myselect = $("select#foo");
    myselect[0].selectedIndex = 3;
    myselect.selectmenu("refresh");
});

$.fn.jqmSelectedIndex = function(index){
    var self = $(this)
    self
        .prop('selectedIndex', index)
    .selectmenu("refresh");

    return self;
}

$("select#foo").jqmSelectedIndex(3);