hundreds of thousands of apps that run on Apple and Android devices will not work on phones like the Lumia 900 that use Microsoft’s Windows Phone software. And many developers are reluctant to funnel time and money into an app for what is still a small and unproved market. So Microsoft has come up with incentives, like plying developers with free phones and the promise of prime spots in its app store and in Windows Phone advertising.

via nytimes.com

I bet less than 5% apps will fall into this category.