Considering they have not released the full android version, and the fact that they can't use any of their existing code -- quite a while. I also think they can't use any of the open source libraries they used on iPhone, Android , etc... because they have to use XNA.

If i were them, I'd wait untill Microsoft adds native support. By the time they finish their game using XNA they might have released native support, and then they're stuck with 2 codebases.