if (typeof (jQuery) != 'undefined') {
    jQuery(function () {
        jQuery.fn.exists = function () { return this.length > 0; };

        if (typeof (jQuery.fn.dialog) != 'undefined') {
            initConfirmDialog();
        }
    });
}

function initConfirmDialog() {
    var cnfrm = '#cnfrm';

    if (!$(cnfrm).exists())
        $(document.body).append("<div style='display:none' id='cnfrm'><p></p></div>");

    $(cnfrm).dialog(
                {
                    autoOpen: false,
                    modal: true,
                    show: 'fade',
                    speed: '200',
                    buttons: {
                        "Да": function () {
                            $(this).dialog("close");

                            if (lcallback) {
                                isp = true;

                                lcallback(lcallbacksender);

                                isp = false;
                            }
                        },

                        "Нет": function () {
                            $(this).dialog("close");
                            return false;
                        }
                    }
                });

    window.confirm = function (mess, callback, sender) {

        //console.log(arguments[0]);

        if (mess == "")
            return false;

        if (!isp) {
            lcallback = callback;
            lcallbacksender = sender; // arguments.callee.caller - not all browsers send valid arguments (

            $(cnfrm + ' p').html(mess);
            $(cnfrm).dialog('open');

            return false;
        }
    };
}

<script type="text/javascript" src="/Scripts/jQuery-UI/jquery-1.7.2.min.js"></script>
<script type="text/javascript" src="/Scripts/jQuery-UI/jquery-ui-1.8.16.custom.min.js"></script>

<asp:Button ID="Button1"
runat="server" Text="Button" OnClick="Button1_Click" OnClientClick = "return confirm('Post back to the server?', function(sender){sender.click();}, this);"  />

var lcallback = function() {};
var lcallbacksender = null;
var isp = false;