I discussed several of the validation and JavaScript/AJAX
improvements coming in ASP.NET MVC 3 with in my blog post about the first ASP.NET V3 preview release.
One of the nice enhancements with ASP.NET MVC V3 is that the
AJAX and Validation helpers in ASP.NET MVC now both use an unobtrusive
JavaScript approach by default. Unobtrusive JavaScript avoids injecting
inline JavaScript into HTML markup, and instead enables cleaner separation of
behavior using the new HTML 5 “data-“ convention (which conveniently works on
older browsers – including IE6 - as well). This makes your HTML smaller and cleaner,
and makes it easier to optionally swap out or customize JS libraries. The
Validation helpers in ASP.NET MVC 3 also now use the jQueryValidate plugin by
default.