Anandtech had access to a 2.66GHz Nehalem processor which they pitted against a 2.66GHz Penryn processor. Performance improvements ranged from 20-50% faster for the Nehalem chip. h.264 video encoding was tested and saw a 44% speed boost on Nehalem vs Penryn on the equally clocked processors. They claim that the 2.66GHz Nehalem outpaces the current top-of-the-line 3.2GHz Penryn.
Apple will certainly adopt the Nehalem processors after they become available. While the first server-grade Nehalem processors are expected in Q4 of 2008, desktop and laptop models won’t be available until sometime in 2009.