After the positive feedback from ASP.NET MVC 2 RC1, Microsoft has decided to release RC2 (Release Candidate 2). This release fixes a few bugs in the RC, but adds one big feature, and that's a change to the validation model. The new feature makes the default validation system validate the entire model. In MVC 1.0 and MVC 2 RC, the validation would only happen on the model that was posted to the server. Brad Wilson has done a comprehensive post about this here.
You can also check some highlights of this release in ScottGu’s blog