The XNA FAQ has an interesting item ...
Q: How exactly can I share my 360 game to other 360 users? Will my game only be available to people with the XNA "Creators Club" subscription? Will it be available to all 360 users that have an Xbox Live account?
A: There is currently no supported way to share binaries on the Xbox 360. Currently, there are four requirements that must be met in order to share a game targeting Xbox 360 which is developed with XNA Game Studio Express.
The individual you are planning to share the game with must be logged in to Xbox Live and have an active subscription to the XNA Creators Club
The receiving user must have downloaded the XNA Framework runtime environment for the Xbox 360
The receiving user must have XNA Game Studio Express installed on their own development PC
The game project, including all source and content assets, must be shared with the receiving user. The receiving user then compiles and deploys the game to their Xbox 360.
Seems like this is going to be a developer only system then whereby you can test your XNA-based game in hopes that it is picked up by someone with the resources to get it onto XBLA proper. Hopefully someday, they will have a mechanism to share binaries. I mean, if they're going for the whole Youtube for games thing, they really have to make a "creator's" creation be available to the unwashed masses or else it will reduce the drive for someone wanting to target the 360.
Don't get me wrong, I will be first in line for a creator's club subscription ... I just hope they figure out the optimal solution :-P