The reason why you can't discount any game being multiplatform between the PS3 and XBox 360 is because their combined sales at the end of 2007 will be (roughly) equal to what people expected the PS3 to sell on its own; also on a worldwide basis (and in most regions) neither platform is strong enough to support exclusive content at the current development cost.
As a basic exercize to demonstrate my point, take major game releases (like MGS, Halo 2 and FF) and calculate the ratio of users who bought these games upon their release; after that calculate the ratio of users who would need to buy these new games in order to match the sales from the previous versions. If I were the CEO of a major corporation I'd be fairly worried about my bottom line if I was releasing a game exclusively for either platform.
