Well XBLA and PSN are both, primarily, P2P networks. I would be surprised if they did dedicated servers for this, outside of the hub worlds.
PC i could see P2P co-op and dedicated for the competitive multiplayer and hub worlds
I see it like this:
1. P2P for the in game stuff, across all game modes and platforms. They will limit the in game numbers to small amounts (very similar to Halo for connectivity reasons). I see this as already confirmed by their statements about presenting local players in limited numbers in the meet up "hub or Miiverse" style sections.
2. Servers behind the scenes to keep that persistent universe and data. Bungie already run a datacentre worth of servers internally and externally. This will just be ramped up for the new persistent universe. It's already up and running and has been for Halo for a long time.
3. Something akin to Join In Progress and Matchmaking is behind the scenes just without menus or visible loading. It's all done with smoke and mirrors now, again I see this as already confirmed in the reveal about their technology experience combining and essentially becoming transparent as if it's not even present.
4. Look at their previous experience such as Firefight, coop, network test gametypes like generator defense or the latency hiding techniques going on in Reach. Also look at their technology in the last 5-8 years of P2P with servers for matchmaking and statistics etc. They are just expanding this to include a story universe with trade exchanges and meet ups.
5. Polish what they already have experience in and send it out to the world as one package.
If you look at their ground breaking CE = console FPS, Halo 2 = online multiplayer/chat, Halo 3 = theatre/coop, ODST = firefight then you start to realise they don't really create new innovations but refine existing disparate technologies into a seamless experience. Bungie are taking this design/development mantra to a new level in terms of no loading times, no menus (somewhat), no visible matchmaking etc. It about making it fully immersive.
Some speculation predictions:
1. All cutscenes and dialogue etc done in game engine, no pre-rendered stuff at all.
2. Most if not all story will come from character in game models/customisation. This will dynamically shape at the backend server end and future Bungie DLC releases design around the dynamic story landscape. I think the most innovation will be in this portion of design/development. A more dynamic version of episodic content if you will.
3. Lockstep will not be present, traditional asynchronous networking will be used. EPIC did this with Gears of War campaign AND horde modes. Could they pull a just cause 2 hack style massive multiplayer? Maybe but I doubt they'll go insane numbers of players based on design and playability reasons alone.
4. Further innovation in letting PSN/XBL/device owners all participate in a single universe across platforms and generation versions too.
5. The genre blurring of FPS, RPG, limited MMO and possible RTS. Again this is the innovative part but it's been attempted many times so more like iteration refinement than outright something brand new.