I'm looking forwards to an updated sensor too, but since t2i, t3i, 60D, 7D all have the same sensor, it would be really awkward to have a better sensor in t4i before they update 60D and 7D....like someone said before, they want the customers to move to higher end...
Canon has shown us in the past that they are willing to use the Rebels to showcase a new sensor, I presume to prove it in the field first, and fine tune the mfg process perhaps. REcent history of the XXD versus the Rebels, in fact, shows that it is more comon for the Rebels to "lead". Examples; While the 30D body was resting on its 8MP laurals, the 400D Rebel appeared with its 10MP sensor, and it wasnt for another year that this 10MP sensor appeared in the 40D. But soon after 40D appeared with 10MP, Canon promptly introduced the 450D Rebel with a 12MP sensor 6 months later, and this "lead" remaind for another nine months, until the 50 was introduced with 15MP.
The 50D briefly lead the 15MP sensor wars for about 3 months before the Rebel 500D followed suit, but then Canon Releaed the 7D shortly thereafter with the 18MP sensor which trickled down, you guessed it, to the Rebel 550D first, and this "lead" (over the XXD) continued for 6 months before 18MP appeared in the 60D.
so it would not surprise me at all to see a new sensor appear in the T4i. maybe it will be 18.4 or something "ish" that can still be called "18MP" but still signal a new sensor