Today Microsoft published the source code of ASP.NET MVC Framework at Codeplex:
We’re delighted to give you an early preview of the first release of the ASP.NET MVC source code on to Codeplex. You will be able to find this project at http://www.codeplex.net/aspnet and it represents the first of a number of planned releases of the source code for the ASP.NET MVC framework as well as some others in the future. Please take a look and feel free to send us feedback about the MVC framework and about using Codeplex for this type of release
The license won't enable you to redistribute your patched version of ASP.NET MVC (Microsoft wants to avoid having multiple incompatible ASP.NET MVC versions floating around and colliding with each other). But it will enable developers who want to get started building ASP.NET MVC applications immediately to make progress - and not have to worry about getting blocked by an interim bug that they can't work around.
Read ScottGu's post for more details...