Sucky javascript.

gistfile1.js

$(document).ready(function() {
	var loginLink = $("a#login");
	var loginWindow = $("div#login");
	var loginForm = $('#login-form-dialog');
	var submitButton = $('#button-logindialog-submit');

	$.fn.submitWithAjax = function() {
		this.submit(function() {
			$.post(this.action, $(this).serialize(), null, "script");
			return false;
		});
		return this;
	};

	loginLink.click(function () {
		loginWindow.html('<%= escape_javascript(render :partial => "checkout/login") %>').dialog(
			function() {
				submitButton.click(function(){
					loginForm.submitWithAjax();
				});
			}
		);
	});

});