I Have reason to believe that Arrendale Processors will not be coming to the Macbook Pro line anytime soon. As you all may know Apple is currently using NVIDIA as its primary chipset. If apple was going to transition to Arrendale (nehalem) processors, they would likely use NVIDIA as their primary chipset. BUT there is a battle between NVIDIA and Intel (because of licensing) and Intel will not let NVIDIA make chipsets for its Nehalem micro-architecture, So if Apple was to transition to Arrendale they would likely have to use Intel's chipset (which is also their intergraded graphics)
or they will have to stick with Penryn as their processors and use keep using NVIDIA as their chipset.