There's a map pack for Halo 3 coming soon, and after that, there's a singleplayer expansion, but after that? Is there anything more planned? Probably not, as Bungie are saying "goodbye" to the game.
Chatting with IGN, Bungie's Brian Jarrard says:
Overall though, we are viewing the ODST campaign and the included multiplayer maps as the final exclamation point in our ongoing efforts to extend the Halo 3 experience. We're really grateful to our fans for standing by Halo 3 for so long and continuing to show us such strong support.
Halo 3 was out in September 2007, ODST will be out later this year, that's a two-year run. Chin up. Be happy, remember the good times, prepare for new Halo - from Bungie or otherwise - and move on with your life.