I would think that eventually, at some time, the extra episodes might come out as an exclusive re-release just for the 360. Think the GTA Double Pack, or the GOTY edition of Morrowind - both had excellent lives (GTADP for Xbox outsold SA).
Provided that the extra content can actually fit on 1 DVD-ROM. The Shivering Isles download for Oblivion was 1gb.
Back from the dead, I'm afraid.