Nokia's success depends far more on the strength of WP7 than does WP7's success rely upon the strength of Nokia.
I hear it's different in the USA, but here in Australia many people would LOVE to buy a Nokia, their brand name is associated with 'quality'. If Nokia can hit the right price points, I can see users who have been disappointed with their budget Android devices heading to the 'safe haven' of a Nokia device.
It will take a while to gain momentum, but I'm optimistic on the outcome. I hope they go well because I'd love to develop for WP7.
Given the pseudo-acquisition of Nokia, MSFT has a golden opportunity here to create an end-to-end appliance the way Apple does; to make sure that they're selling a sexy package, instead of selling a sexy OS in blah hardware (i.e., all of the WP7 launch phones) or a blah OS in sexy hardware.
They can finally do the vertical integration that IMHO is sorely needed if anyone hopes to compete at Apple's level.
Buuuuut given MSFT's track record in recent years, and with the departure of some key heads (who pushed for vertically integrated "experience"-building, instead of OEM licensing), I'm not banking on it happening this way.