Microsoft’s Ray Ozzie and Scott Guthrie, here at Mix07, have announced the addition of a cross-platform .NET runtime within Silverlight, Microsoft’s browser plug-in for streaming video and rich internet applications.

I’m not sure this counts as a major announcement – after all, this very thing was talked about here at Mix last year. What is new though is that you can download the alpha from today, for Windows and Mac. Nothing has been said yet about timing, but I guess the fact that this is described as alpha implies a significant wait before a final release.

Nevertheless, this is a critical development for Silverlight, particularly as Adobe’s Flash now has a JIT-compiled Javascript runtime that in language terms is not that far removed from C#. Differentiation may be mainly in the runtime library that is available, and of course Visual Studio integration.