unless the multiplatform release is of significantly worse quality on one platform then I don't believe that is the case. All of this is speculation of course but especially due to the makeup of this gen multiplatform is best for most games since the ps3 version will maximize Europe and Japan while the 360 version will maximize America sales. Castlevania Lord of Shadows will probably reach 1million units and according to vgchartz be the best selling castlevania ever. Enslaved is just not worth it as a product considering it can be rented and beaten 3 times over in a week.
Really the only way exclusive makes sense nowadays is if the game is first party published or the publishers make a good deal with sony or microsoft to keep the games exclusive. The majority of consumers don't care if the title they buy is exclusive or not, but might buy a certain system if their most wanted games are exclusive to that platform hence why we are seeing very few exclusives not published by microsoft or sony.
Making a game exclusive will make it sell better on the one platform but in my opinion except if the port is very poorly done, will not let it outsell both sytems combined.
