var selected = $('input[name=selectid:checked]').val();//若未被选中 则val() = null
if(selected == null){
alert("未选中！");
}else{
alert("选中！");
}

var n = $("input:checked").length;
if(n==0){
alert("未选中！");
}else{
alert("选中！");
}
$("div").text(n + (n <= 1 ? " is" : " are") + " checked!");
3：checkbox数组全选、取消全选
$("#selAll").click(function(){
$("input[name='cbox[]']").attr("checked",$(this).attr("checked"));
});