I see no reason why multiplayer could not be cross-platform other than MS/XBOXlive and/or PSN denying it. The only thing needed to be sent over the wire is coordinates for characters their statuses etc which should be identical for ANY platform.
Just wanting x-platform multiplayer is not a valid reason to wish for a single console market.
3rd Party publishers would have to discuss options with both MS and Sony to get this to work. I doubt that they will be on board for this as there is no incentive for either one of these companies to allow this to happen. If you want to play a game with your friends you will try to persuade your friends to buy the game for the system that you both have or try to persuade them to buy a particular console. IF 3rd party games were x-platform then this incentive would be lost.
