ScottGu writes a detailed post about the preview. See an excerpt below. You can download the bits here.

ASP.NET MVC 3 is compatible with ASP.NET MVC 2 – which means it will be easy to update projects you are writing with MVC 2 to MVC 3 when it finally releases.  The new features in MVC 3 build on top of the foundational work we’ve already done with the MVC 1 and MVC 2 releases – which means that the skills, knowledge, libraries, and books you’ve acquired are all directly applicable with the MVC 3 release.  MVC 3 adds new features and capabilities – it doesn’t obsolete existing ones.

ASP.NET MVC 3 can be installed side-by-side with ASP.NET MVC 2, and you can install today’s “Preview 1” release on your machine without it impacting existing MVC 2 projects you are working on (they will continue to use MVC 2 unless you explicitly modify the projects to retarget them to MVC 3).  When you install “Preview 1” you will have a new set of ASP.NET MVC 3 project templates show up within Visual Studio 2010’s “New Project” dialog – choosing one of those when you create a new project will cause it to use MVC 3.

Below are details about some of the new features and capabilities in today’s “Preview 1” release.  Unless otherwise noted, all of the features I describe are enabled with the preview build you can download and use today.  More ASP.NET MVC 3 features will come in future preview refreshes as we flesh out the product more and iterate on your feedback.