Microsoft have just released Beta 2 of Game Studio Express. It's the SDK which helps you make games for Xbox 360 Homebrew, and Windows.
I think it's fantastic that they're opening up the console to homebrew developers, and also giving so much help for people to get started. The one thing which irks me is that it only supports C#. Professional game development is currently C++ based, and I don't see it changing for a while. C# is a great language which is easier in a lot of ways than C++. It's great if you're just getting started, or you don't care about non-Microsoft platforms, but if you're looking to get into games, remember to work on your C++ too. Having an Xbox360 demo, but not knowing C++ well, is not as good as having a Windows only demo which is written in C++. C# is good to know too though, as it can be used to quickly develop tools.