Back in February, Microsoft and Nokia announced a partnership, which would see the two companies working together to take on Apple and Google in the smartphone market. At the time, Nokia CEO Stephen Elop it’s now a “three-horse race.”

The companies have now signed their agreement, ahead of schedule, allowing both parties to begin engaging with operators, developers, and other partners on the endeavor.

“At the highest level, we have entered into a win-win partnership,” said Elop. “It is the complementary nature of our assets, and the overall competitiveness of that combined offering, that is the foundation of our relationship.”

The “third horse” will involve Windows Phone-based Nokia devices, and will include a new ecosystem to accompany that (yet another ecosystem for developers to worry about).

“Our agreement is good for the industry,” said Microsoft CEO Steve Ballmer. “Together, Nokia and Microsoft will innovate with greater speed, and provide enhanced opportunities for consumers and our partners to share in the success of our ecosystem.”

It will also involve the integration of numerous Microsoft offerings into Nokia phones. These include Office, Xbox, Maps, and of course Bing.

“Microsoft will provide Bing search services across the Nokia device portfolio as well as contributing strength in productivity, advertising, gaming, social media and a variety of other services,” the newest announcement says. “The combination of navigation with advertising and search will enable better monetization of Nokia’s navigation assets and completely new forms of advertising revenue.”

Bing’s share of the search market has grown every month since its launch. This partnership could be huge for expanding it even further.

When Microsoft announced Windows Phone 7, we suggested that it could be one of Google’s biggest threats, and one of Bing’s best chances for significant growth. Lackluster sales of the initial devices kind of played that notion down a bit, but the Nokia deal could breathe new life into it.

“Mobile is critical to Bing’s success, and providing the best possible search experience on Windows Phone 7 is an important part of our strategy,” a Microsoft spokesperson told us in November.

Bing also has the Facebook partnership working to its advantage, both providing web results in Facebook, and providing Bing users with Facebook integration from the search engine – an area Google continues to shy away from (Blekko is also deepening its Facebook integration, by the way).

It will be interesting to see how the social media element of the Microsoft/Nokia partnership plays out as well.

The companies intend to ship devices next year.