An Appcelerator and IDC study finds that Microsoft's Windows Phone 7 mobile OS has moved "decisively" ahead of RIM's BlackBerry OS to become the clear number three mobile OS behind iOS and Android. According to the study, Windows Phone has gotten a big bump from Nokia, especially in Europe, while interest in Blackberry has "collapsed."

The study, which surveyed over 2100 mobile developers, looks at how companies are making the move from web to mobile, and how mobile is fundamentally transforming customer relationships.

"Windows Phone 7 separated from the pack to become the clear number three mobile platform this quarter," a summary of the report reads. "The OS climbed 8 points to 38 percent of respondents saying they are ‘very interested’ in the platform, the highest ever for Microsoft."

To put that number in perspective, 91 percent of developers are interested in the iPhone, while 83 percent are interested in Android handsets. Only 21 percent are interested in Blackberry.

"Microsoft is enjoying symbiotic success with Nokia," the summary continues. "When asked why developers are more interested in Windows Phone 7 now than a year ago, a plurality (48 percent) said it was the Microsoft/Nokia partnership. Nokia also received high marks from its new Lumia Windows Phone 7 smartphone announcement last month, with 28 percent of developers saying they are 'very interested' in developing for the device. This is more than double the interest in Nokia's own Symbian and MeeGo OSes."

RIM, meanwhile, has experienced a sharp drop-off in developer interest, with BlackBerry OS phones dropping 7 points to 21 percent, according to the study. "There’s now more interest in Nokia's new Lumia Windows Phone lineup than RIM's smartphones," the summary notes.

Interestingly, Android experienced a bit of a drop as well, with interest in Android phones dropping 4 points to 83 percent. Interest in iPhone is unchanged.

Thanks to Michael T. for tipping me off to this study.