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 deploythem to the XBox. And I'm pretty sure these new apps are not XNA based.
XNA with the Creators Club allows anyone with the right skills to develop arbitrary software for the Xbox with games delivered through XBLIG. Developers who like to do things by the book can enter into a formal agreement with Microsoft (I've signed the forms myself when I toyed with the idea back in 2007) where you can submit your software to XBLA.
The "XDK" and special developer-edition consoles are only required if you want to be targeting the native Xbox console - all you need to do is set-up a bona-fide games development company (even "indie" companies with a handful of employees) and apply to Microsoft. Assuming you pass the audit and stump up the $30k needed to join the programme then you're all set.