well I am doing exactly the same thing now.
I'm currently going through the painstaking process of just manually converting all the code to c#. there is no little saviour like the Android NDK here with winmo7, you HAVE to use c#
if I had my time I would and WILL definitely look into something that converts from c++ to c#, it is completely unrealistic to try and manage a multi platform project across multiple languages.
depending on your app: silverlight I believe is meant for the more 'Applicationy' type apps, where XNA is meant for games (or 3D apps), but I think both are coded in c#
EDIT: lol, sorry skipped over the part about how you were porting an openGL game, definitely use XNA, converting from openGL to XNA(directX like) will be the least of your worries, its fairly strait forward. its converting the code that's the pain. XNA is meant for n00bs writing stuff from scratch, and for them, it is awesome. to that end, it is good if you still have all your asset's source: hopefully still having your max or maya model files, and tga/bmp/png texture source files. if so, the content pipeline will automatically convert textures, and for models, converts .x or .fbx files exported from max or maya.