Bungie have revealed that it has plans to produce more Halo 3 content, or support ODST for a while at least. Despite previously saying that ODST "was the final exclamation point in our ongoing efforts to extend the Halo 3 experience".
Producer Allen Murray told Joystiq the following:
Halo 3 took two and a half years to make, and we're still working on it. We'll probably be working on it for two more years.