The ZMan:
Right now there is no way to sell your game on any platform other than windows and there are no announcements on if this will even be an option on the 360, though we are all hoping for the ability.
Actually the ability to publish games to XBLA (not XNA Community Games) was discussed at last year's Gamefest. It was announced that their plan to publish games to XBLA all with a retail console fell though. However they were planning to make it possible with the use of a devkit. Basically you can program 95% of your game using a retail 360, you just need the devkit to finish it, adding stuff like leader boards, achievements and true skill matchmaking.
However at the time nothing was ready. I asked about it at GDC08 and I was pleased to find out that their XBLA plan was still in place and not overshadowed by Community Games. The XNA Extensions as they are called were supposed to be available for companies with Dev Licenses near the end of May. Meaning they should be available right now, but its not the type of thing they announce to anybody who isn't an official 360 Developer.
The newly posted Gamefest 08 overview talks about the fact they will be holding sessions about XNA Extensions. I expect we are going to hear a whole lot about it next month.