Here is the
Halopedia interview with Joseph Staten which has a timeline quote that may be old news to most librarians:
Basically, Halo 3: ODST happens while the Chief is away from Earth - after he leaves for the "Delta Halo" in Halo 2 and before he crashes into the jungle at the start of Halo 3. ODST doesn't cover all the events of Halo 2 and Halo 3 but it does fill a very interesting part of that gap.
On the other hand, it should probably be considered a primary constraint when merging the games' timelines.
