It was announced today that Nokia will partner with Microsoft to use the Windows Phone 7 OS software as the foundation for their mobile ecosystem, which ultimately means that Nokia gives up on maintaining Symbian. It is certainly a big step for Microsoft since Nokia has a pretty decent market share among smartphone manufacturers and it will now add up that percentage to the WP7 platform, given that the sales and device adoption keeps up with the current trends.
Ultimately, Nokia was moving towards this event a while ago. With mass adoption of iPhone and Android devices, lots of developers tried to focus on larger user groups with more development opportunities. Both iPhone and Android flourished with thousands of applications while Nokia's own Ovi Store stayed behind. I used one of their devices before and really didn't see any major titles released for it (if I would have to compare it to other smartphone giants). The development for Symbian wasn't extremely popular either, so it was a logical step to adopt someone else's platform and keep the hardware. That way, Nokia will not be responsible for device-specific development tools (although at the moment this specific point is unknown). Unless, of course, they decide to force some platform-specific libraries to be used specifically for their devices. But this is less likely since Microsoft usually doesn't create vendor-specific developer tools on top of the existing set.
Even though Windows Phone 7 will be bundled with new devices, I am really curious about the reactions of developers that invested resources in Nokia's Qt to develop platform-specific applications. What they will most likely have to do now (in order to keep up with Nokia's software) is to switch to .NET and adopt Silverlight and XNA as the primary frameworks.
As a side note, there was an article on ComputerWorld stating that Google was also competing for the spot of the software platform carrier. There is no official statement/reaction from Google at the moment of writing this article.