The easiest way to encapsulate Halo 3 ODST's multiplayer disc is to call it "Halo 3 Multiplayer Complete." That was our takeaway from a meeting with Bungie at GamesCom today. Discussing "the other game" in the ODST box, the Bungie rep reiterated, "It's all the multiplayer maps we released with Halo 3, then all of the DLC we've released since then, along with three new maps."
The standalone disc pretty much means that precious Xbox 360 hard drive space can be cleansed of Halo 3 maps ... of course, there's always the option of putting the whole disc on the HDD as well. The Bungie rep explained it best by saying, "It's like putting in Halo 3 into your Xbox, except it doesn't have the campaign."