I'm having this problem too. it played Halo 3 fine for awhile, but when i tried to load up campaign this weekend in anticipation of reach, i couldn't play.
today, i'm playing reach, and i got all the way to the load screen for mission 6, and it gives me the DRE. i've cleared the cache, removed the hard drive, and flipped from horizontal to vertical with no change.
i'm also using the white xbox with the grey tray