Microsoft team is posting the source code for the unit tests that they 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.
You can download it here if you want to start using it immediately.
However, the full version will include more features (and incorporate feedback people provide on this week), deliver a more seamless installer, provide nice Visual Studio integration, and deliver up to date documentation.