ExecuteResult is a method in the System.Web.Mvc.ActionResult class. Are you sure you don't want to look there?
http://aspnet.codeplex.com/SourceControl/changeset/view/23011#266522
The Model property is simply an object type and has always been AFAIK.
, , , , Ajax.BeginForm :
<% using (Ajax.BeginForm("Customers", new AjaxOptions { UpdateTargetId = "customerList"})) { %>
<% } %>
<div id="customerList">
<% Html.RenderPartial("CustomerList"); %>
</div>
'UpdateTargetId' - MVC "" ( , InsertionMode AjaxOption InsertBefore InsertAfter) Id .
Ajax, IsAjaxRequest, , :
if (Request.IsAjaxRequest())
return PartialView("CustomerList");
return View();
, !