@Maddus Mattus: Nowhere.
Lets be clear, these "Apps" which show up in the "Marketplace" on the Xbox are developped by priviledged companies AFAIK. There is know way (currently) to get your own apps on there.
You still need a very expensive special Dev Xbox to write XNA games and deploy them to the XBox. And I'm pretty sure these new apps are not XNA based.