If they ever release a HD-DVD enabled 360, I'm predicting that it will be a monumental flop:
1)The 'average joe' consumer couldn't care less about high definition movies, and will have no interest in a higher priced 360.
2)The home theater buff consumer will shun it because of the lack of "next-gen" (Dolby Digital True-HD, DTS-HD, DTS-MA, multichannel PCM) capability on the 360's part, as well as the fact that the stand-alone hd-dvd players will probably be cheaper.
I suppose it would also depend on exactly how much more it would cost - if it was, say, $30-40 or so more, it might work...but if they choose to add close to the price of the add-on to the console, it won't work.