Note: I don't care about CoD4 multiplayer. Everyone's been raving about it, but why do i want to buy a game just for the multiplayer? If i want to do that i'd buy a fucking MMORPG, not an FPS.
Some people like SP some prefer MP. I think for years everyone gloated over the SP in a game. If some game came out that was say MP only it would get scrutinized.
I think today there is a vastly larger audience of folks who buy games for the mulitplayer aspect, not just for the SP. Some folks could care less about the SP portion. I think most publishers under estimate the market for people who like playing good mulitplayer games. They've been so focused on eye candy console like games for the last several years.