In ASP.NET MVC 1.0, we shipped Ajax helpers implemented as extension methods on the AjaxHelper class (and available via the Ajax property of your views). In the box, we shipped two category of Ajax helpers: Ajax links and Ajax forms. Both fundamentally did the same thing: make an asynchronous request, and do something with the result when you’re finished (including knowing whether you got back a success or failure response from the server).
In ASP.NET MVC 3 Beta, we've updated the runtime to enable a feature we're calling "Unobtrusive Ajax". We have also created a consumer for these unobtrusive Ajax attributes that uses jQuery to perform the Ajax requests on our behalf.