you're so right I should have studied that more carefully. Even in the case of the 15MP sensor, one could argue a virtual "tie" (the 50D/500D intros were very very close), but still the history just doesn't show Rebels leading the way. So the present intro is without historical precident!
Well, here's the thing. Technically, the T4i/650D sensor is "new" in the sense that it has on-sensor phase-detect AF pixels. Fine. But it's still an 18 MP sensor. While gear-heads and technophiles abound here on CR, the general public will see a Rebel/xxxD with an 18 MP sensor just as the past two generations have had 18 MP sensors, and conclude it's the same sensor (which, from an image capture standpoint it may very well actually be). So calling it a debut is practically incorrect - from a consumer perspective, it's the same sensor. The question then becomes, will Canon debut what is popularly perceived as a 'new' sensor (i.e., more MP than the current one, or some other major and tangible improvement) in the 70D, or the 7DII? Since the 7DII will sit above the 70D in the lineup, it probably makes the most sense to debut the new sensor in the 7DII...the 70D gets the same sensor as the T4i/650D (i.e. AF during video is touted) and the 7D's AF system and various other small improvements. Maybe.
I too find it extraordinary that this new sensor apparently makes no contribution to image capture IQ -- it appears to be more about hybrid AF. We have postulated all along that in order for it to appear "new" Canon would need to either bump the MP count or engage in a marketing thrust to re-educate the public as to why the new one is better. So far, the low noise marketing is aimed Digic 5 in-camera jpg processing which may very well be the (stills) target audience for this camera. So they are touting low noise in ways that most entry level photographers wont really know how to take advantage of, but will be thrilled to know it is there.
I suspect and hope that the 7D2 public is diffrerent, and that Canon realizes this 18MP sensor in a 7D2 would be boring. x-vision has rightly pointed out that the highest performing 1.6x sensors tend propogate out to the rest of the 1.6x line, unless of course a lower-entry level body lags behind. But here is where the historical precident fails us: xxxD's rarely "lead" but 7D "always" leads! at least we know that Canons sensor fab shop is not closed...