16:9 and DD 5.1 are part of the HD spec, as well as the resolutions of 1080i and 720p. Yes, you are playing at high resolutions, but not HD. High Definition is not a qualifier as in it is higher res than other things. "High Definition" is rather the specific spec that has been mentioned. Many call 1080p High Definition, but it is something different and a higher resolution. There is no HDTV broadcast(true HDTV is broadcast) in 1080p
1280x1080 is a high resolution, but not HD. That doesn't make it bad or lower than HD. to play at a High Definition(again, not a measure of greatness, but something specific) set your resolutions to 1080i or 720p in the game and you will experience HD in the game.
Keep in mind, we've all been using 480i televisions if you're in the US. Actual broadacasts in 1080i or 720p are incredible in comparison. I know the numbers themselves aren't impressive in computer terms, but as far as a broadcast goes, they are absolutely incredible and are close to the limit of what the human eye can distinguish. Sports are night and day, and action dramas take on new dimension in 5.1 surround. Again, not impressive in computer terms, but keep in mind that progressive scan DVD is only 480p.