This week's update (which you can download here) includes a number of improvements to ASP.NET MVC.  Some of these include:

In addition to posting the source code for the ASP.NET MVC framework, we are also posting the source code for the unit tests that we use to test it.  These tests are implemented using MSTest and the open source Moq mocking framework.  A VS 2008 project file for the unit tests is included to make it easy to build and run them locally within your VS 2008 IDE.

Significantly easier support for testing Controller classes.  You can now unit test common Controller scenarios without having to mock any objects (more details on how this works below).

Several nice feature additions and usability improvements to the URL routing system (more details below).