If I am seeing 1333Mhz in "About this Mac" then I believe that the memory is reporting itself as 1333Mhz. Systems operates fine with 2x2GB existing modules and 2x4GB new Corsair modules. Activity Monitor reports I am using over 4GB means that at least one of the new modules is being used.
Since 1333MHz memory would be backwards compatible on a 1066Mhz bus, I am guessing that Corsair could package a higher spec memory module as 1066Mhz.
What could go wrong if I go by above assumption.