Bungie has said all downloadable Halo 3 maps will be included with ODST later this year, including the upcoming Mythic map pack and the three exclusive expansion levels.
The Bungie website says all standard Halo 3 multiplayer maps will be stuffed inside ODST, too.
This makes Bungie's decision to add no further content to Halo 3 after the Mythic Map Pack and expansion ODST more palatable.
We've been treated to two map packs so far: Heroic and Legendary. The Mythic Map Pack will be a free extra for people buying the collector's edition of Halo Wars on 27th February, before being released as DLC on Xbox Live Marketplace shortly afterwards.
Bungie also released a free Cold Storage map on 7th July last year.