Not sure have you tried the component cables? What type of TV?

No answer, but you are not alone:
http://forum.teamxbox.com/showthread.php?t=553209


>I have the Elite hooked up with HMDI to my TV that is supposed to support 1080p. What the heck is going on?
>
>>My 360 does 1080p over component with no problem. It is the old model with no HDMI.
>>
>>>Rod,
>>>Does your 360 offer 1080p? I try to set it up on mine and 1080p is grayed out, so I had to select 1080i. I am using an HDMI cable and my TV supports 1080p.
>>>