March 2013 is the prediction, still in the Early 2013 trimester of 2013. If Apple releases an iMac in September, and they adhere to their previous 8-10 month lifecycle, then by month 6, it will both be (a) early for a refresh, and (b) running out-of-date hardware. Not that the iMac hasn't been guilty of this in the past, but still; odds of an iMac announcement coinciding with this month's iOS-fest is slim to none. So, if they do it in either October or November, that just means that the iMac will be using up-to-date tech for much less of its refresh life cycle than if they had just waited.
Mind you, Apple has done this in the past with the Mac mini; it's not inconceivable that they will do this with the iMac and the Mac mini this time around, especially with such an increased emphasis on Mac notebooks and iOS devices.