I think a Demo Channel for DS games is already in the works, but I don't think that Nintendo will release a Demo Channel for Wii games, as the files will be large and a hard-drive may be required.
well they are going to have a demo channel called Everybody's Nintendo Channel but there is no release date set yet but i have heard in november or december 10th.
There is one in the works that will be released extremely soon. It is unconfirmed whether the channel will allow us to actually demo a game, but it is assumed that it will simply show trailers of a game, nothing more. It will be used to show trailers of wii and ds games and will be released 'in November.'