Nokia sold about 26 million smartphones the last quarter. Let's imagine that only *half* of that people will get a Nokia Windows Phone 7 in Q4/11 then WP7 will be available on about 16 million phones (13 million Nokia + about 3 million HTC/Samsung/LG/Dell/...) per quarter. With these numbers WP7 soon will be the next big competitor to iOS.
But why would people buy a Nokia Windows phone? Nokia used to sell Symbian phones, and for many years (until around 2007) they were the best you could buy. Plenty of people kept buying them, although in shrinking numbers, when better phones became available. But now there are no Symbian phones anymore. Since all these Nokia customers have to switch to something different, why would they switch to a Windows phone? Most of them use Windows at work, they know how bad it is, and they wouldn't want it in their private life if they got paid. The huge majority of these people will switch to iPhone or Android.