A lot of people feel like the ASP.NET Postback model is awkward and hard to work with. If you've also been using non-.NET frameworks like Rails or CakePHP then you are probably even more frustrated with it. The closest option so far has been Castle, which appears to be a nice framework but for many MS oriented shops is too big of a leap, not just politically but because of its almost complete replacement of the rendering model.

Fortunately, MS has recognized the growing momentum and interest in the MVC approach for web apps and is releasing their own take on it. MS's tendency to always release their own version of popular open-source tools is usually irritating but in this case I think the approach is a big enough change that only MS can get the typical .NET-only shop to try it out. Additionaly, they are adding better support for using existing ASP.NET pieces and controls than Castle could.

Jeffrey Palermo, on of the leaders of the ALT.NET movement that has popularized the use MVC and TDD in ASP.NET has a positive overview of the new Framework in an article at CoDe Magazine. Check out "Use the ASP.NET MVC Framework to Write Web Apps without Viewstate or Postbacks" here.

Microsoft's leader of Developer tools, Scott Guthrie, also has this overview or even better, this video.