I've just gone out and bought an XBox 360 premium. I have plugged it in and selected the HDTV option switch on the cable. However when I try to change the settings to 720p or 1080i, my screen just goes blank and then returns back to the default 480.
My TV is a Samsung 42" Rear Projection that is "1080i Ready". So why does it go blank when I change the setting?
It looks like the problem was the Samsung TV could display 1080i and 720p, but only at a refresh rate of 50hz. The Xbox outputs to those resolutions at 60hz, thus the TV can't display it. Only thing you can do is try doing all the updates for the Xbox and hope something there gives you more options.