Microsoft has clearly stated they want apps in the app store. Project Siena is proof of that. Look out app store, here they come. Microsoft's Project Siena is a beta release of a new technology for app enthusiasts to allow you to create apps, right from a beautiful and powerful Modern UI app.
It's a Metro app, that creates apps.
Microsoft's Project Siena is live in the App Store now. It's available in the Windows 8 store as well as for RT. It's pretty easy to create simple apps. You can get into more difficult app creations very easily and if you know anything about Excel formulas and calculations this will be very easy for you. Remember that it's still a Beta app. There may be a few things missing like publishing directly to the app store and some more difficult dynamics within apps. It's coming.
Get the app here.
One question I still have it is whether or not you can create RT apps but I'll hopefully answer that soon. You can, however, build an app in RT. The app is available for both RT and Windows 8. Enjoy, and drop your questions and creations into the Surface Geeks Forums. Go get it now and get busy! Stay tuned for more information, demos, and cool apps coming from SurfaceGeeks.net!