I upgraded to HD from Comcast a few weeks ago. I received a new HD box (Motorola DCH-3200) to be able to get 1080i programming. However, when I plugged in the HDMI cable into the TV and cable box, the cable box would always power on, go through the power on self-tests, and then lock up, displaying DUI / DU1 in the display. This, I found out, is the digital display of the word 'DVI'. I googled the error, and only found a few people who experienced it, although there was enough information to infer that this appeared to be some sort of handshake issue between the components. Since HDMI is a digital spec (complete with copyright management -- yay), there is more than just a signal output from the box to the TV. The two have to speak with each other before it will work.
I'd seen issues with people putting a receiver in between the cable box and the TV, and that causing issues because the receiver didn't propagate the signal correctly between devices, but I had a straight connection. However, after much fussing and the arrival of the Comcast technician, we did try an alternate HDMI input on the back of the TV, and that worked like a champ.
I have no idea why another input would work. There is no mention of different specs for the various HDMI inputs on my TV, but all I know is HDMI1 doesn't work, while HDMI2 does. Incidentally, HDMI1 works with my DVD player, just not with the cable box. By the way, I have a Phillips LCD (I forget the exact model number).