1080i and 1080P are not supported by the game. look on the back of the box!
I'm sitting here with the box in my lap,
HD Video Output
480P+720P+1080P
You should probably look on the back of the box yourself.
I'm trying to figure this out myself, because I know that the game was displaying in 1080 when I got it, and then it just stopped. 720 zooms you a lot closer to your character in 3rd person, I can only see him from the waste up, wereas in 1080 I saw almost his entire body in 3rd person and my view wasn't obstructed with my character taking up a quarter of my screen space.
I don't understand what's going on. If I take 720 out of the display resolutions available it will display the game in 480.
The only thing I can think is that Bethesda removed 1080 support to correct some of the lag problems people were experiencing.
I'm really confused and annoyed with this, and would love to know what's going on.