只是为了阐述redsquare的答案，你需要你的jQuery改变这个...

Just to elaborate on redsquare's answer, you would need to change your jQuery to this...:

$(function() {
    /** 
        ...What ever else you might have onload... 
    **/
    $("a[name='logout']").live('click',function() {  // Now using the .live() event
        $.post("logout.php", function(data) {
            $("li.login").html(data);
            bind_login_submit();     // <---- Notice, this is new
        });
    });
    bind_login_submit();    // Call this function to initially bind the submit
});

function bind_login_submit() {
    $("form#login").unbind('submit').bind('submit',function() {
        var usernameVal = $("input[name='username']").val();
        var passwordVal = $("input[name='password']").val();

        $.post("login.php",{username : usernameVal, password : passwordVal}, function(data) {
            $("li.login").html(data);
        });
        return false;
    });
}