It's unclear whether this means that they'll more DLC, expansions or whether it's simply talking about technical support.
At GDC this week, Bungie told Joystiq: "Halo 3 took two and a half years to make, and we're still working on it," producer Allen Murray said. "We'll probably be working on it for two more years."
According to Bungie's GDC panel, the company is also working on several non-Halo projects.
The ODST expansion is due for release this Autumn, but if you can't wait until then for more Halo 3 content you can look forward to the Mythic Map Pack, which will launch on Xbox live on April 9, 2009.