i have no doubt that valve would have included some sort of multiplayer if they could have. but portal is a puzzle game, and would be very difficult to build a multiplayer around. it would be like tetris having a team deathmatch mode. but, fortunately, portal 2 does not need multiplayer. it is a monnument by its self. the co-op is some of the most fun i have had on the ps3 in along time. if valve ad