The first Nokia phone to run Windows Phone 7 instead of Nokia’s long-running Symbian platform is reportedly already in development.

Reuters reports that Nokia’s CEO Stephen Elop confirmed the project is underway. It’s a quick turnaround considering the mobile giant just announced its new Microsoft partnership about one month ago.

It is a hugely symbolic move for Nokia, which has been making mobile phones for decades, and from day one has only loaded its phones with its own proprietary operating systems.

Its Symbian OS was so popular that at one point, it licensed the software other mobile phone manufacturers. And then there was the iPhone and Android, which have turned the industry on its head. All of the legacy mobile platforms seem outdated and uninteresting. In addition to Nokia, Blackberry maker Research in Motion has been hugely affected.

But moving away from Symbian, one would assume Nokia would choose Android. When Elop announced the Windows Phone 7 partnership, many looked at his past work history as a Microsoft executive as biasing his decision. It was later reported that Microsoft paid as much as $100 million for the deal.

So, the first Nokia phone to run Windows software maay be the most important phone in Nokia’s long history. It is expected to be released in 2012.