Here are the details,
Will be a stand-alone retail title with full 1000 gamerscore.
Acts as an expansion to Halo 3's campaign, showing the events on New Mombosa.
You will NOT be playing as Master Chief, but as a simple ODST trooper.
The Mythic Maps will come included but will also be available earlier via XBL (for a price).
The campaign will not be available via XBL, only via the retail disc.
Players using the normal "Halo 3" disc will be able to play online with those using the "Halo 3: Recon" disc.