What do we need, and how do we go about it.
First of all, an xbox (360 atm) build would require a full rewrite of the old Direct3D code. We have been building exclusively for OpenGL for the last year, but evolutionary there is no real reason not to build for DirectX. Many of us have extensive experience with developing DirectX applications, going back as far as DirectX 3. What I want to prevent is that we would move too far into Direct3D at the expense of our OpenGL advancements. This is nothing more than an idly made political decision so it is open for discussion.
In practice, in order to build for Xbox, we need to deploy XNA game studio on our development machines, which will allow us to pull a repo branch and compile that for Win32 and xbox at the same time. XNA lies on top of Visual Studio and the .NET Framework 2.0.
There are a few different flavors of XNA game studio. There is the Express edition which is free but I don't advise using that to build production code because there are some issues with it: to use the developed software on your Xbox, one needs to join the "XNA Creator club." Otherwise, you won't be able to fully build, test, and share the software. XNA creator club has a $99 (US) annual subscription fee. This Xbox live subscription will provide the additional pieces needed to develop non-commercial XNA Framework-based software that work on an Xbox 360.
Moreover, in order to distribute your software, the people who use it will also need to be members of the XNA Creator Club, and as such will also have to download the XNA Framework and have XNA Game Studio Express installed on their own development PC.
This is because the people you are sharing your software with must compile it and deploy it to their Xbox. In essence, you are sharing only your source code and they are compiling their own copy.
There is something else to note, and that is the fact that OK, you can install XNA and develop games for PC and Xbox, but you can only distribute PC games commercially, which is sort of what we are doing with XAudio2 already now. My point is, that if you actually want to distribute a game for box, you still have to talk to Microsoftabout licensing.
If any of this information is incorrect or outdated, feel free to post or PM me on it.