Personally, I don't think that Bungie will ever stop making anything Halo related. That would be like Nintendo saying they were through making Zelda games, the community would riot.
I personally think it can go either way after Halo 3. I would like to see a continuation of the story in some way, whether it be what happens to the Master Chief and Cortana, or the condition of the Elite's homeworld. And if they don't want to make more, that's fine too, they left on an EPIC note. I think what would be awesome is if they made a fourth Halo game in which you switched back and forth between Master Chief and Arbiter, just like you did in Halo 2.
Just like a few other posters said before me, NEVER say never!