The problem with this is that Nokia has invested a significant amount of resources into its own systems already. If it partners with Microsoft for WP7, Nokia will just be another player in the market with not a whole lot to distinguish itself from other manufacturers, such as Samsung.
Nokia won't be able to ship Windows Phone 7 updates for existing Nokia smartphones, so everything will hinge on how many new Nokia WP7 devices they sell over the next year. If they don't start selling better, it's no net change...
There needs to be uniformity in the OS though. I don't think more than 3 can co-exist in the long-term. Users don't want too many options in the OS, but the hardware options is a huge plus.
It will be interesting to see if Microsoft can use Nokia's international presence to tap into the developing European smart phone market.
Hardware ooption is not a huge plus if you see for performance. Why Apple's products are working so smooth is because of the dedicated hardware.