My bet is ... they're just using the same sensor as in their current 18MP APS-C cameras such as the 7D ...
Quite likely the same cell [photosite] design as in the 7D, 60D, 550D and 600D, since there is no newer cell design in any Canon DSLR except the very different 1DX. But in a new smaller sensor: I doubt that Canon is just covering up the extra 4mm of sensor area at the sides with black tape!
Things change fast if the sensor technology in the four models atop Canon's current EF-S DSLR range, including one model from 2011, is now "out of date" and behind Panasonic in the "ISO race"! (*)
(*) To me, the ISO race is getting as silly as the MP race: striving to reduce IQ problems that are already far less noticeable than with the equipment used to take most great photographs of the 20th century ... or even the previous decade. I think that most of us, from family snap-shooters to professional photographers of sports and news events, will get more from the "focus accuracy and speed race".