If they do split, it will have a LOT more to do with than somthing simple like "Bungie doesn't want to make more Halo games." Because if Bungie didn't want to, but MS offered a 100's of millions of dollars, well of course Bungie would change their minds. And of course MS would offer that if they -had- to.
So if they split, it will have to do with other issues. And I don't think there could be any other huge issues that MS couldn't "buy" their way out of to change Bungie's mind.
I conclude that their partnership continues. I would guess there may be a new contract on the line, and who could blame Bungie for asking for one, but nothing more.
And, to be honest, I think Bungie will continue to make Halo games. Maybe not sequels, but more Halo games. Let's face it:
A) Bungie makes a ton of money making Halo games.

Bungie remains one of the top dev. teams in video gaming as long as they make Halo games.
C) Bungie enjoys making Halo games and Halo fans.
I don't want to see Halo Golf, or Master Chief Kart, but I'd love to see:
- Halo XBLA titles. Something along the line of a Prince of Persia XBLA or something that was a Halo XBLA series would be great!
- New Halo titles that are 100% multiplayer without a campaign.
You don't have to have sequels to make more fantastic Halo games.
Anyway, I am really getting tired of this Bungie/MS split rumor. It's been 4-5 straight days now that news sources are reproducing this rumor as an article and it's getting old. I would like to know the truth but I really don't see them splitting ties. That would be like the New York Yankees moving to a new city. It just won't happen.