I think it's a safe bet to say that we all have heard about Halo 3: ODST. We know about its story, its background and its gameplay. In fact I'm pretty sure since it's a Halo game that there are plenty of people out there who know every detail they could possibly get their hands on about the game. So what was discussed at the PAX panel on the game? All that stuff of course, but the devs also revealed that ODST would feature a fully developed sub-story that is separate from the main plot of the game.
Bungie has been working with Fourth Wall Studios to make "Sadie's Story," a secondary story that will be presented within ODST about the life of New Mombasa resident named Sadie during the final hours before the city is destroyed (as occurred in Halo 2). Not much was revealed about the story except a few flashes of images, but the story was written by Bungie lead writer and ODST Creative Director Joe Staten and Fourth Wall, and they both collaborated with well known comic artist Ashley Wood.
Other than a brief glimpse of concept art for Sadie, there was no hint given to how the story would be presented in the game. It was revealed, however, that the work done on "Sadie's Story" doubled the amount of voice acting the studio had to do for ODST and that you'll be be picking up snippets of the story as you play through the main game as the rookie. "Sadie's Story" will also have connections to ODST's main story and the Halo universe as a whole. It sounds like Bungie is putting a ton of effort into something that many people might not even see if the story is optional. Makes that $60 price tag seem a bit more valid.