I think Windows Phone 7 is so different from Symbian or Maemo that switching to that OS will be a big financial gamble.
Will satisfied Maemo or Symbian users buy a Windows Phone Nokia in the future? I think that a lot of them will not, hence a big gamble.
Producing phones based on WP7 in addition to Symbian (and possibly MeeGo/Maemo) is much more likely option.
WP7 is different enough and the microsoft brand might provide Nokia a way to grow in the US martket.