Since the company came together in 1991, Bungie have worked on five Halo games: Halo 1, Halo 2, Halo 3, Halo 3: ODST and Halo: Reach. Will they ever do a sixth? Probably not.
In an interview with CVG, Bungie senior designer Lars Bakken has said in response to the "this the last Bungie Halo?" question:
In terms of a platform for Halo games made by Bungie, yes. I can't answer that question for Microsoft though. Whether they have something in the works or not, I don't know if they're actually ready to talk about it yet, but in terms of Bungie-produced Halo games, ODST and then Reach is probably all you're going to get.
It's a little ambiguous, leaving open room for another "expansion" for Reach similar to what we're getting with ODST, but then, it's also mostly clear: that's it for Bungie and Halo. Time to move on, water under the bridge, greener pastures, you get the idea.
Let Microsoft worry about exploiting expanding the franchise.
Halo Reach to be Bungie's last 360 game [CVG]