It's a fair point but if previous years are anything to go buy even if they announce mid Sept it won't be in the shops until at least a few weeks after and even then supply will be constrained. In order to affect the Q412 earnings they would really have to announce and release it late August, which is always possible. Either way you would know the release date with plenty of time to assess the likely impact on Q412 earnings.
However I don't think they will do that. I think Tim Cook is happy to defer much of this revenue to Q113 in order to take the heat out of the share price, stabilise things for a while and then boom start fiscal 2013 off with a bang. That quarter would include new iPhone, new iPods, iPad Mini, 13" Retina MBP. Presumably they will try and stagger the release dates around the world to manage supply/demand and to defer some of the revenue into Jan/Feb/Mar to boost Q213 earnings ahead of the next iPad