ClickOnce support?

I don't see why not. On Windows, the XNA assemblies are just that, assemblies.

I think the automatic-update features of ClickOnce would be extremely helpful. Xcopy and MSI don't give you this ability, and ClickOnce makes a lot of sense for simple "Flash" type games.

I like being able to build the stand alone MSI's. This is one reason why I would like the express plugin available for the retail versions of Visual Studio. I've not had much reason to experiment with ClickOnce yet, however I have read a few articles on it.