Black ops: Playing without the map pack

The system tries to match you up with similarly "ranked" players with the same available map packs. If it can't do that, it will try to match you up with players with a subset of the same available map packs. All of this is automatic and seamless. I have two 360's and for a while one of them didn't have the map pack installed. I didn't notice a lack of games. My friends don't have the map pack, they join my party and we play as usual. No special hoops to jump through.

Right now it appears there are seperate playlists for those with and those without the mappacks.

As to rather this will change once they release more mappacks or anything its hard to tell right now.