im pretty sure radiant (cod level editor) is coming at least for multiplayer. When i open the console in mp and type for example /devmap mp_lolol it says that it cant find mp_lolol in the /usermaps/folder. that means that its possible to launch custom maps in mp, but in sp?
If the same modders will start modding as in cod 5 then trust me, WAIT FOR THE BIG !!!
I would assume some of the modders will switch over, and if I could get into it, I would make some pretty awesome maps. Unfortunately the only in depth map maker that I ever became skilled with was the one for Black Hawk Down.