Don Reisinger wrote an article on wether or not there should be another Halo game anytime soon. Here are my thoughts on the subject.
I can see Don's point about how advantageous it would be to pair a very generation of Microsoft Console with the newest Halo title. The thing is that there really isn't a reason why they can't release new content for the existing game. If the expansion content is quality then it doesn't diminish the brand. If it does, that could be a problem, but I don't think Bungie has released enough "stinker" Halo games yet to soil public opinion against the franchise. I mean they are coming off one of the largest entertainment (not just video game) releases ever.
I would like to point out that I'm nether a 360 or PS3 fanboy as I own neither console. I favour the PC, but this isn't a pro PC rant in anyway. I've tried Halo 3, and found that I don't really care for it. However, I would root for it as I like seeing gaming as an industry thrive.
If I do have one concern, it would be with the fact that the newest game will be a prequel, and we all know what can happen to prequels of beloved Sci-Fi series. Just ask Jar-Jar or Capt. Archer.