I also have a Hitachi 53SDX89BA and I bought it with the same assumption as you: that it's HD ready. The specs in the manual states the following: "Full 1080i HDTV Capatable with Set-Top Box". I beleive this means that it does not include an HD tuner, but that it is capable of handling up to 1080i resolution. Normal (non-HD) resolution on this TV is 480i.
So here's my problem: Until recently, I wasn't using the HD settings on this TV because I don't have a set top box, nor do I subscribe to any HD content from my cable provider. However, I just purchased an XBox 360 along with an XBox 360 HD-DVD player (it's an XBox peripheral that works with the game console). The Xbox asks you to choose the resolution output (480i, 720i or 1080i) to match your TV's HD capabilities. When I select 1080i output and change my TV's settings to receive an HD signal, the picture quality is amazing. However, the video signal seems to get lost ocassionally (i.e. TV picture gets scrambled) but then it comes right back. This does not happen when I select a 480i output on the Xbox...but of course, the picture isn't as crisp and clear as a 1080i picture.
I don't know if this is a problem with the TV or with the Xbox. Has anybody else had this problem??