What's on the Halo 3: ODST disc is all Bungie Studios has planned for the game. In a post-release interview with the team, Bungie confirmed there are no plans to release any downloadable content for Halo 3: ODST. The developer's focus has moved onto Halo: Reach, a much more ambitious project.
"There are no current plans for any ODST DLC [downloadable content]," said design lead Lars Bakken. "The Bungie team is full steam ahead on Reach with a lot of work between now and the multiplayer beta and then final release next fall."
Disappointment shouldn't last long, as we're expected to learn more about Halo: Reach soon.