Blazon:What I ultimately want to see is a LIVE Community Games channel that is available to both Windows and the XBOX 360. Games that are designed to work with both platforms, or just one of them, would be marked appropriately.
I think ultimately, the XNA team would love to see 100% feature parody for every platform. I have absolutely no knowledge of what is being planned, but I'd assume a Windows flavor is at least on the drawing board for both XBLA and XNA.
Off hand, I can think of one large barrier to XNA Community Games for Windows Live: non-standardized hardware! It is extremely difficult to develop Windows games that run uniformly across a wide range of machines in terms of performance and capabilities. The XNA Framework helps lessen the incompatabilities, but there is still a lot of problems which can arrise such as having fall-back shaders and the need for dynamic level of detail. That's not even to mention driver bugs and other anomolies.
When I was preparing the Racing Game starter kit for release, we submitted the game to a compatibility lab and there was a reasonable range of odd configuration specific problems which needed to be fixed. Independent game developers simply do not have those resources. Other teams at Microsoft are working to make this easier. Some examples include the capability standardization in D3D 10 and 10.1 as well as Vista's "Experience Index". Again, I don't have any clue what work is being done (if any) for community games on Windows, but I'd suspect the work is dependent on these future simplification regarding machine performance and capabilities.