The halo.bungie.org forum currently features a couple of very informative posts from Matt Soell. Here are a few snippets:
You're not gonna be able to run around the whole ring, mathematically-generated or not. Too many ways that could go wrong, especially in net play. All the environments in Halo will be human-generated, but we still need to figure out how players will get from place to place on the Halo.Right now [the Warthog machine gun's supply of bullets] is, for all intents and purposes, infinite. But that's because no limits have been imposed yet. There will be limits.
The real question you seem to be asking is how much control players are going to have over the drop ship or their actions within it. The answer, right now, is not a lot. You probably won't have to fly it anywhere; there will almost always be vehicles better suited to the tasks you'll have as a player. Thought has been given to letting players control turrets on the ship, but that's not definite yet.
Check out both threads for more.