From my experience it runs fine on Vista and XP and for a professional usage.
I'm just going by what Microsoft say. XNA is not supported on Vista. I'm glad you're not experiencing any troubles. this is especially great for small developers who want to make a step into the console world and build prototypes or complete games which they can offer to publishers.
You can do that without XNA. If you contact any publisher at all, inquiring about a deal, they will tell you to submit a prototype that runs on DirectX. If they like the game they will either a) front you the money for a proper SDK, team and other development tools. b) Buy the rights to it, and sub-contract it to a more experienced developer. This will be addressed in XNA Pro, were no details have been announced yet.
Cheap shot, I know, but if no details have been announced, then how do you know? You can access all the hardware!
No you can't. XNA does not give you access to any hardware. XNA gives you access to .Net wrapper objects for DirectX which interfaces with the drivers who manipulate driver data. Where does it say you cannot share your game?
The last five lines.