Except if you believe the rumors that Mac Pro's are getting custom CPU's
Well, even that doesn't make too much sense. A custom CPU would still most likely be based on existing architectures (i.e. Westmere or Sandy Bridge) so nothing special. A higher clocked Westmere part would be my guess if I had to make one. Given that Mac Pro doesn't exactly sell like hot cakes, it makes no sense to develop a new architecture just for Mac Pro, considering that developing an architecture costs hundreds of millions.
IMO those custom CPU rumors were hoax anyway. The source of them was a site that has never published any of their own info, they simply re-post news published by AppleInsider, MacRumors and so on.