That's still a client-server model, only one of the clients happens to be the server as well. Quake 1 worked that way, too. The Halo games use a client-server model but support transparent host migration if the original host quits mid-game. That's really tricky to get working even halfway decently!
The Call of Duty games all ultimately descend from the Quake 3 code base, so I wouldn't be too surprised if MW2 and BLOPS were still using a recognizable variation of the networking model described in the article by Brian Hook in my link.
That was great for folks away from servers, like me - until the guy decided to disconnect, when a new host had to be chosen.