Think about this for a second. Why would they do the latter? You have to ask yourself some very real questions and then the answers make the probability of the latter occuring almost nil.
1. If the system could be done as you described as the latter, why haven't they implemented this backend for the consoles? This decreases probability that what you hope for is going to happen.
2. Why has this never been done before? It is essentially "dedicated" if IW is hosting the servers. It costs them money and since we know money is the root of all evil where evil = Activision, this is a pipe dream to expect. We wouldn't be having such an uproar if IW were hosting the servers. It's an uproar because it's P2P, we're going to have to piggyback on John Q. Lame's shitty connection because he's downloading a GB of porn simultaneously.
3. This is P2P only. About as much as they're going to handle on the backend is the master server, connecting players to each other. They're not hosting anything meaningful, trust me. It doesn't fit within their financial attitude.
In short: This is just the same as MW2 on the ranking side. You have P2P matchmaking where all players hop on someone else's connections and it brings all of the pain associated with it. Rampant cheating, host dropping out and shitting all over the game, poor connections, hacked matches, the game is going to be a nightmare.