It's no "great risk." It's about as dumb as a strategy can get. And on top of it all, characterizing it as a choice is at best prevarication. No other OEM makes an exclusive choice of operating systems.

Nokia could have, and probably should have run Windows Phone, Android, and Meego on their smartphone hardware. And the likely result would have been that WP would not sell well compared to the other choices.

EDIT: The parent post here sounded just a little too pat. But I didn't check until after I had posted a response. Yup. It's this first post even for this user. Hello Nokia PR!