As it's been said in other threads on here, there's nothing at this moment in time for Apple to upgrade to.
Intel have announced a delay with the montevina chipset until July '08, and judging by the length of time it took apple to get the Penryn in computers, it's likely to be August before they update again.
What I'm hoping for is an updated Mac Pro, but then I'm not expecting that to be for a while yet either...