That is correct. As long as your game does not go down and access any of the windows OS functions then it should be easily portable. I think in the new MonoGame 3.0 99% of the XNA framework has been reconstructed. I ported a older game I made to Mac OS X with MonoGame 2.5 and it worked well .
In my case I used XNA as a 'toy' to gain some knowledge, and then I moved to a 'naked' API. I chose openGL. I really don't care right now what will happen to XNA, but if something new appears I will definitely check it out. I would suggest as people above, if you want to stay with C# go SharpDX, MonoGames or SlimDX.
I'm currently in this position, using XNA to gain sufficient knowledge before moving on to something else.
I wanted to ask about Monogame... its libraries have same structure as XNA? Meaning that I can freely convert my projects to Monogame once content pipeline arrives without modifying too much to become compatible?
Thanks. I'll then stick with XNA for few years until I gain sufficient knowledge about everything I need... and then switch to Monogame, and after Monogame to C++ and DirectX/OpenGL.
I'm just saddened XNA is coming to the end like this... it was API that enabled me to enter this "world" without having to study for multiple years in addition to faculty studies and studies for what I currently know. And, it was one of (rare?) Microsoft products that actually worked, regardless of background behind it XD