The thing on the back of the box shows what resolution the game is compatible with, not what it will actually display at. If your TV wasnt capeable of displaying 720 but ONLY 1080i then thats what it would display at. However since 720p is the preffered display resolution for 99% of the PS3 games out there, the game will auto-choose it should your TV be capeable of showing it.
Besides... 720p is better than 1080i, so it shouldnt be a huge issue for you ;]
No offense but that's total baloney. Ico/Shadow Of The Colossus runs at 1080p, and the game doesn't "choose" 720p.
I've also run FF XIII and GTA 4 at higher resolutions than 720p.
I figure it was intended at some point, but probably when they saw the lag the game has at even 720p they canned the idea, but the boxes were printed.
Nice selling point though. Got me to buy it for the 1080i. Someone at Bethesda should run for Congress.