Long story short, N9 video processing power is about average for modern smartphones. It does not match the high-end power you'll find on the current iPhone or top-of-the-line Android phones. And so, it does not play HD video very well (if at all).

If you need your phone to play video at the same speed and quality as a 40 inch HDTV does, you need to pick one specialized for that task. (So far, the HTC top-of-the-line phone, called "Rezound" in Verzion's lineup, is the only one that has a native 720p screen resolution, and so is the first one I'd actually consider decent for playing real HD video.)

Exactly as bad as 480i looks anywhere else. The N9 has absolutely no trouble with 480i video.

Remember, you're asking your phone to become a high-definition television set. The hardware needed to do that (and do it well) has only become widely available in the last year or so. The N9 just doesn't have the latest bleeding-edge GPU silicon...