Taken from http://blogs.msdn.com/briankel/default.aspx...
You've heard the rumors, now hear it from the source... This morning we announced the Microsoft XNA Framework. The XNA Framework will bring a unified set of game development class libraries to Windows and the Xbox 360. This means that game developers can write games which shares a great deal of source code and even source game assets (sound, models, etc) between the Windows and 360 platforms. Oh, and did I mention it's all managed code? That's right, we are building a custom implementation of the Common Language Runtime for the Xbox 360 as well.
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More details at our Web site - http://www.microsoft.com/xna - including a link to the press release which went live this morning. It's admittedly sparse on details at the moment, but we'll be sharing many more details later this summer at our Gamefest event. This is just the beginning...