Well, I went from T1i (15MP) to 7D (18 MP) to 1Ds2 (16 MP), so I'm obviously comfortable in that area, but as long as we're wishing...

The new 70D and T4i/T5i are supposed to be nice and sharp APS-C bodies, so lets assume we can just take that pixel density and make a FF sensor that big. 18MP * 1.6 * 1.6 = 46.08MP.

As for the difficulty of getting large high density sensors with any sort of reliability, couldn't Canon make the sensor a bit smaller, and place them in a 2x2 gapless grid? Or is placing sensors with that sort of precision out of reach. I'm remembering IBM spelling IBM out of atoms a decade ago... surely we can align a couple sensors... right?