Microsoft announced it now has 15,000 developers creating applications for its mobile operating system, Windows Phone 7, with around 3,000 applications available today.

It’s a good sign for the infant mobile operating system that had around 13,000 developers earlier this month. A 15 percent bounce in a few weeks isn’t too bad, given that it’s going up against powerhouses like Google’s Android mobile operating system and the iPhone/iPad operating system, iOS.

Windows Phone 7 finally brought Microsoft’s mobile operating systems into an age that is dominated by apps. The Apple App Store already has around 300,000 of them, and the Android Marketplace has around 100,000. Instead of the operating system being core to the experience of a phone, like Windows Mobile’s traditional strategy, it has since become just a springboard for the user experience that applications can deliver.

Clearing several orders of magnitude to catch up to the rest of the smartphone market might seem daunting at first. But it’s also worth noting that Windows Phone 7 has a few cards up its sleeve that the iPhone and Android operating systems don’t have. Namely, it can connect with Microsoft’s Xbox Live online gaming service and promises to beat the pants off its competitors in that sector. With more than 25 million Xbox Live users, Microsoft and its partners can effectively create the phone of choice for hardcore video gamers.

Microsoft is also able to integrate its incredibly popular Office applications pretty seamlessly into a mobile interface. Android and the iPhone operating system don’t have immediate access to the well-known document editing office application. The closest thing is DataViz’s Documents to Go. But that company was acquired by Research in Motion, which quickly killed support for the budding WebOS mobile operating system. So the future of that document editing application isn’t clear on competing platforms. That means Windows Phone 7 is in a good position to make a play on the enterprise market if developers can jump on board with apps that integrate with Office.

Numbers clearly aren’t everything in this game. But after seeing the number of developers ramp up for Windows Phone 7, Microsoft employees are probably throwing each other a few fist bumps.