An imminent update to the XNA Framework will allow for sharing of packaged XNA Game Studio Express Games.
Developers will be able to package their binary games into a single file to share with other users of XNA Game Studio Express. These files can be emailed or hosted on websites like any other files. To run a game, you just double click a file and it will unpack to your Windows-based PC or Xbox 360 console.
Now XNA can really enable the "YouTube of Games."
Check out the other announced features in the upcoming XNA update in the XNA Team blog.