Since it was announced that H3: ODST's campaign would allow players to play as several different ODSTs throught the game, my thoughts are that the Reach campaign will play out in a similar manner - also think Halo 2 with Arby and MC.
Having not read any of the Halo books, my info might be a little shaky but my understanding is that MC and a few others are in orbit and go to a ship that didn't initate the COLE protocol. The other Spartans went down the to surface to protect the ODP generators. So, my theory is the missions will skip from one group to another, giving us the full picture of what actually happened on Reach and allow us to fight in several different battles.
Following that theory, I'd also think the last mission will be played as MC and will involve him getting Linda's (I think) body to the Autumn before Keyes makes the jump.
I hope that's how it plays out because you'll get to play as different Spartans, all with different abilities, learn some new character's stories and see the entire fall of Reach - and maybe see a few Spartans die in the process.
Thoughts?