Halo 3: ODST Campaign Edition available now
Sony has been aggressive in experimenting with re-selling its legacy content. From MAG toStarhawk to, most recently, Uncharted 3, the company has been open to selling games "piecemeal," with the single and multiplayer functionality sold separately. Microsoft is sort of copying that approach with todays release of Halo 3: ODST "Campaign Edition," available for $14.99 on Xbox Live Marketplace.
According to the Games on Demand listing, the Campaign Edition "does not include the Halo 3competitive multiplayer functionality or maps." Theres no separate multiplayer-only version of the game to purchase on XBLM, either. (Halo 3 is available on XBLM for $20, complete with single and multiplayer.) The CE does offer Firefight in addition to the campaign mode, however.