It not a channel, but rather an input that needs to be changed.

Look on the right side of flat panel TV's. You will see a button called INPUT. Push that and you will rotate through all available inputs. Plug in your HDMI and it works, with audio.

It is possible that the hotel TV was ordered with certain inputs not activated, maybe to slow down theft.

An iPad 1 or 2 will send out 1080p if the content is in 1080p and you use an HDMI cable. Otherwise you often get a "picture in a box" effect where you can't fill the screen.

I understand that new iPad (3) has a native resolution greater than 1080p, so you should fill the screen - if the TV understands the input signal. I need to play with one to be certain.

Apple TV, RoKu and Boxee will also work on most Marriott TV's once you change the INPUT button.

Hotels do keep custom remote controls in rooms. Maintenance probably added a channel that requested HDMI input.