For the last 2 decades MSFT's whole company strategy is to get developers to make apps for Windows Desktop. Name the last big "Windows App" that made you buy a copy of Windows? The web killed that and iOS and Android made it worse.
Now Microsoft needs devs to make Windows 8 apps and they are hoping that the inherently huge Windows Desktop ecosystem will make it so cheap to port to mobile that developers will build tablet and mobile apps from their Windows 8 codebase.
The only problem is if you have a native app you are talking about 3 or 4 codebases to deal with - iOS, Android, Windows (and its variants), and the web. As it stands now most devs will stop after doing iOS, Android, and web. Who knows if Windows App Store is going to sell enough software for it to be worthwhile to add a 4th codebase.
The opportunity in this is for shops who are willing to port popular Apps to Windows. At this point the only apps that are worthwhile to build Windows first are games.