Today OWC confirmed that the Mac Pro's processor is indeed upgradeable, successfully replacing the default Intel E5-1650 V2 6-core 3.50Ghz processor with an Intel E5-2667 V2 8-core 3.30GHz processor with 25MB of L3 cache, an option not offered by Apple.



From MacRumors:

The upgraded processor gave OWC's machine a 30 percent multi-processor performance boost, outperforming Apple's standard 8-core option with a Geekbench score of 27004 vs. 24429. With a replaceable CPU, customers can purchase more affordable lower-configuration Mac Pros that can be updated in the future as processor prices drop. Prices for multi-core processors today remain high, with the CPU OWC used from Intel priced at 2000. Apple's own CPU upgrade options range in price from 500 to 3500.

  Mac Pro CPU Upgradeability Confirmed With Processor Swap