In a conversation with Gamasutra Microsoft exec Shane Kim let slip that Microsoft and Bungie are working on two episodic, downloadable titles for the Xbox 360, one of which is set in the Halo franchise:

"We're in the design phase now", continuing: "The first series will be set in the Halo universe."...The game collaboration was first announced at the Microsoft X06 event, when the first was explained to be "a collaborative effort with Bungie Studios to co-create a completely new and original chapter in the Halo franchise." However, this is the first time that the series has been confirmed to be in episodic, apparently downloadable form.

I wonder how they're planning on fitting a next-gen Halo game into that 150MB download limit -- maybe you'll have to buy a disc at retail and then download the episodes online?