In ASP.NET MVC 2, we shipped both client- and server-side validation support. The client-side validation that we included in MVC 2 was a custom validation system written against ASP.NET Ajax. We also included an experimental version written against jQuery in the MVC Futures project.
In ASP.NET MVC 3 Beta, we’ve updated the runtime to enable a feature we’re calling “Unobtrusive Client Validation”. We have also created a consumer for these unobtrusive client validation attributes that uses jQuery and jQuery Validate to perform the validation on our behalf.