Microsoft has barely ever announced that they would discontinue a technology. They usually let it die progressively by not making any more updates, not providing support, no more activity from MSFT employees on dedicated forums...etc.
So XNA is not dead of course, as It can still be used for Windows desktop, WP7 and XBOX360 platforms, which is already pretty nice, but it is definitely a technology of the past for Microsoft. For the open source community, as you mentioned, there is another story with MonoGame, though at some point, they will have to introduce some new features that were not accessible from XNA (Geometry shaders, tessellation, compute shaders...etc.), and they will have to add new APIs that won't be anymore a XNA thing, It will be a MonoGame thing.
If you think that there is a XNA team working underground, well, there isn't. Not a single job career on Microsoft jobs on the past year has been published about such an underground project (this kind of thing are usually easily spotable).
If you search for a XNA developer from the XNA team responding to technical question at the "Xbox LIVE Indie Games Forums" you won't find any of them for the past 12 months.
But now, if you ask for DirectX and C++ for Windows 8 Metro or Windows Phone 8, you will find several people from Microsoft working on it and helping on official Microsoft forums.
It has been pretty clear from last year Build Conference, that the current focus for Microsoft is going back to native C++ and raw DirectX. XNA is absolutely no more developed at MSFT.