Olympus Japan just issued an official statement. It says that the demand of the lens has been higher than expected and therefore not all that preordered the lens will get it in time (late November-early December). The lens has still an ETA of December 7 at Amazon US (Click here). But a couple of 43rumors readers luckily told me that their US preorders already shipped! And there is even a first lens auction on eBay US (Click here). So I guess the supply shortage will only affect those of you that will preorder the lens now.
Preorders: Olympus M. Zuiko Digital ED 12-40mm f/2.8 PRO Lens at Amazon, Adorama, BHphoto, AmazonDE (viaDL), Amazon ES (via DL).