Well, this seems like the general trend. Cellphone manufacturers no longer make the OS, but rather get the OS from another company, after all these modern phones are almost like PC’s so why wouldn’t it evolve like the PC market? You don’t see HP making their own OS, or Microsoft making their own computers. Soon you will be able to decide what OS you want on your phone when you buy it. This is true even today, as many manufacturers offer phones with different OS’s.
Apple is unique in this aspect, and will probably continue to be so.
But I think Nokia is just following the trend. It’s a correct decision for them to make, considering the situation they are in. But they shouldn’t have locked themselves in with just one OS vendor, however, they probably got a sweet deal on Windows Phone 7 OS. In the short term it might save them, but in the long run it might hinder them.