At the moment, the Xbox LIVE on Windows presentation that Avi Ben-Menahem delivered is still the best source of information:
http://channel9.msdn.com/Events/BUILD/BUILD2011/PLAT-756T
The following link should be active in the coming weeks, and will have more information on the benefits and application process for becoming an Xbox LIVE on Windows developer:
http://go.microsoft.com/fwlink/?LinkId=228422
Quick answers to your specific questions:
1. The Xbox LIVE on Windows SDK will be separate from the Windows LiveID SDK.
2. PinballFX is not currently available in demo or source form. If you're looking for a good tutorial on building Windows 8 games, see the Marble Maze sample and documentation content:
http://code.msdn.microsoft.com/windowsapps/DirectX-Marble-Maze-Game-e4806345
http://msdn.microsoft.com/en-us/library/windows/apps/br230257(v=VS.85).aspx
The APIs used to build DirectX/C++ apps should be familiar to those used in the XDK. You can watch the DirectX-tagged BUILD presentations for more detail here.
There will be sample code for Xbox LIVE on Windows APIs available with the SDK. Avi's presentation detailed usage of those APIs and outlined the major differences from those used on Xbox today. This code should look very different to code you've used in the
past to access Xbox LIVE.
3. This is the bet forum for Xbox LIVE on Windows questions in the near future. As noted, we will have much more information available at the link above soon.