Where can I find information with regards to what is and is not possible with regards to developing 3rd party applications and data services for the ArcGIS Marketplace? Apologies if this is the wrong forum but I did not see a better choice.
I did read the slides for the Road Ahead - ArcGIS Online session from the recent UC. Starting with page 11, it has a few slides on the Marketplace.
The first step to putting apps in the Marketplace is becoming an Esri partner. All apps and data services will be made available via ArcGIS Online items called "Listings". All of the standard ArcGIS Online services will be available for your apps but I image you will also want to use your own hosted services in your own Organization or Developer subscription as well.
Stay tuned for more details in the September time-frame.