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The future of the Windows Phone Store is WinRT, particularly due to Universal apps, which will run on both Windows Phone and Windows desktop/tablets.
Microsoft is therefore eager for developers to port their Silverlight apps to WinRT, and has made a number of resources available to developers to make it easier.
Unfortunately developers will know that there are many APIs available in Silverlight but not in WinRT, which means many apps remained chained to the older technology.
See a video discussing the topic above, and find many links to related resources here.