I'm about 124.940% positive that Rare didn't make Halo, Fable or Gears... Wasn't this discussion about Rare?
Although not at the head of the credits, Rare has been involved in a lot of assorted Microsoft games, and is heavily credited in Halo 3.
I don't think Rare's going anywhere. Their focus has shifted mostly to interface-related stuff on the Xbox 360, and developing the platform as a whole. They were involved heavily in the Kinect. They've gone from the game-makers to the ones supporting the game-makers. It may not be the most glorious job, but I think they'll pull a pleasant surprise on us eventually once they have the time.
Here's a bit of a useless fact I learned at a panel involving them: Viva Pinata was initially a Windows Mobile title, but never released to that platform. It balooned into a 360 game when the higher-ups showed interest. A new version later showed up on the Nintendo DS, which in terms of hardware, is very similar to what Windows Mobile runs on.
You'll notice a lot of Microsoft titles on the Nintendo DS overall. A lot of them are published through a 3rd party, but they're out there. Rare is apparently working on some 3DS titles, which pretty much means that Winmo ports or simultaneous releases would be easy to pull off. Dunno if it means any exclusives in the future, though.
Still, what I wouldn't give for a Kameo sequel...