Why would that be a win for B&N? Nokia got a lot more for ditching it's own OS. And my Nook Color has always been smooth, both with its original software and after having Cyanogen installed. It now runs Nook, Kindle and Google Books very well. I can't imagine WP7 would be an improvement.

Besides that, you forget the e-paper model, which is selling very well - WP7 can't run on those.