Way I see this there were two possibles that lead to this.

Elop came in with this in mind from the start and the board hired him to do just this.

Elop came in looked at where the programmers were with Symbian and Meego and realized it was in worse shape than anyone knows with no hope of getting better anytime soon and decided the only hope to save Nokia was to take this big punt.

I think there are may other paths he could have taken to right the ship including building a WP7 and Andriod phone to gain some market share and get back into the US, while continuing to try and develop the Symbion and Meego platform to come up with the Halo phone they need.

As I see it now Nokia are just an also ran that will never be a major player again.