And no, they aren't copying the Wii. XNA is a game-creating program and from the release of it they had plans to bring these games to the Xbox. If anything, Wii saw this and "copied" the 360 before they got a chance to release their's.
Now that the Wii fanboys have been told off... This will be a similar system to XBLA. But, users upload their video games, and peers review it and such and play. Some demos are already out for free to download of games. There isn't news as to if these games will be free or not (but since it's Microsoft...).
Nothing too fancy except for the announcement of it here:
10:55 - 11:03