As others have commented, in my opinion Windows Phone will have a negligible influence on the mobile market going forward. I think it would be a much better investment of your time to start developing on either iOS/Android (or both!) as early as possible.
Good luck!
Unfortunately Microsoft have almost no experience at doing such a thing, they have always lived in their [big] Windows eco-system bubble and that may very well be the thing that stops Windows Phone, Windows Tablet's etc from ever becoming a real player in the market.
The next year is going to be critical to Microsoft's long term success as a platform provider outside of PCs.
It's a matter of time before people realize they can use Xamarin to port a Windows app to iOS/Android in a matter of days.
Already starting on Android devices :)