For me personally, who’s already using jQuery along with ASP.NET, the main difference will be the intellisense annotation support for jQuery. This will be a welcomed enhancement of my development environment. Needless to say the asp.net team at Microsoft will also build ready-to-use controls in the ASP.NET AJAX Control Toolkit that builds upon jQuery.
According to Scott Guthrie there will be a free download with the jQuery Intellisense support in a few weeks. The ASP.NET MVC download will also contain it and the jQuery library.